Why do some Christians interpret Daniel's 1260 day prophecy as days and not years?

As you have proposed, there is a large body of Biblical interpretation that taps the well known "year for a day" (symbolic) principle that was used in the 70 week prophecy of Daniel. Those who believe this tend to favor applying a restrictive view of "hermeneutical consistency" to other related passages as well. But, there is another sizeable school of interpretation - the "literalist" view - that holds to a literal (and less restrictive) view of interpretation (i.e. interpreting things "literally" first unless otherwise suggested by the context of the passage). Those who advocate this view, consider there to be no such rigid requirement for "hermeneutical consistency" because BOTH (literal and symbolic) principles are used together, with purpose and flexibility, throughout the Bible. In addition, there are weightier considerations to be used as a guide as well - such as whether one takes a "futurist or historicist" view of Bible prophecy.

As above, others hold to a "literalist/futurist" view, not only for greater flexibility and fidelity to scripture, but precisely because of the multiple terminologies that you cited - "As background, 1260 days is equal to 42 months (42 x 30 days), and is equal to a time, times and half a times (360 + 360 x2 + 180 days)". These time periods are rendered in multiple ways (not just as "days"). In fact, those and other similar but related day spacing (1260, 1290, 1335, and 2300 days) all fit together exactly when applied to the Hebrew "appointed" feast days (when spaced 3 1/2 or 7 years apart). In so doing, God's "appointed feast days" can also be viewed prophetically as the necessary framework for God's future "end times".

Below is a link to a (literalist/futurist) timeline that I researched few years back. I made it to illustrate a "hypothetical" 7 year timeline between the two total solar eclipses in the US (2017 and 2024) and the astronomically precise "Revelation 12 sign" (which formed on September 23, 2017) - i.e. by incorporating them as "signs in the Sun, Moon, and Stars". I currently view the relevance of the "Revelation 12 sign" as potentially being the actual "initiation signal" for the end-time tribulation period - where it serves as being a true "sign" in the Sun, Moon, and Stars (using a literalist/historicist/futurist interpretation). More recently, I recognized the need to revise it further, extending it to include the "Middle East" eclipse of 2027 (total eclipse of Aug 2, 2027). It has now become a 10 year period - by adding a three year "gap" into the middle and extending it to Yom Kippur of 2027. However, the previous 7 year version still serves to illustrate the flexibility and use of the multiple elements of interpretation together - "literal, historic, futurist, and symbolic".

hypothetical Daniel's imeline

Why do some Christians interpret Daniel's 1260 day prophecy as days and not years?

Whether a day is interpreted as a literal day or as a year depends on the context. Interpreters looked at the 70 sevens of the Daniel 9 and almost unanimously decided it cannot be literal days, but it could be years. (However, some interpreters duck the whole issue and go for an indeterminate length of time.)

With the 1290 days (Daniel 12:11) and 1335 days (Dan 12:12), though, the context suggests actual days. How could an individual patiently wait for the coming of 1335 years? (Dan 12:12)

Daniel 8:14 says 2300 "evening-mornings": some think that means 2300 days, and some, such as William Hendriksen, think it means a total of 2300 sacrifices, half in the morning and half in the evening making 1150 days.

Hendriksen then says this is the exact number of days that the sacrifices stopped in the days of Antiochus Epiphanes, though where he gets that information I do not know.

On the best evidence, the Temple was desecrated on the Seleucid Calendar date of 15 Kislev 145 (1 Maccabees 1:54) which is 6th Dec 167 BC, Julian date; and was cleansed on the Seleucid Calendar date of 25th Kislev 148 (1 Maccabees 4:52) which is 14th Dec 164 BC, Julian date.

This actually amounts to 1104 days (not 1150 days).

The conversion of Seleucid Calendar dates to Julian dates is taken from "Babylonian Chronology - 626 BC to AD 75" by Richard A. Parker and Waldo Dubberstein. See also "From Abraham to Paul - A Biblical Chronology" by Andrew Steinmann, page 217.

A time, times, and half a time

"A time, times, and half a time" (Dan 7:25, 12:7, Rev 12:14) is introduced in the book of Daniel. It is seen again in the book of Revelation in figures relating to three and a half years, such as "forty two months" (Rev 11:2, 13:5) and "one thousand two hundred and sixty days" (Rev 11:3, 12:6). These are all symbolic references to the same time period, where, it is true, each month is spoken of as having thirty days.

What does it mean? The best explanation I know is summarised here:

The three-and-a-half symbol has a structure of one, two, and a half. What the "one time" represents (a period of tribulation), the "two times" also represents except that there is a doubling, intensifying or prolonging. Likewise the "half a time" represents the same thing again, but there is a limiting or cutting short. https://www.simplybible.com/f919-revln-three-and-a-half-symbol.htm

All that needs to be added is that it is our Lord God, our loving heavenly Father, who will cut short these periods of tribulation. These periods are not outside of God's control, but he is allowing them to fulfil his purposes. The blood of the martyrs is the seed of the church, and the sufferings of this present time are not worthy to be compared with the glory that will be revealed in us (Romans 8:18).

Sometimes "a time, times, and half a time" is converted to days. So in Daniel 9:27 there is a week made up of two periods of three and a half days, and in Revelation 11:9-11 we have two references to a period of three and a half days.

In all cases, whether for years or days, "three and a half" is a symbolic period which refers to a period of suffering or persecution. It may also refer to a period of chastening of God's people for particular sins.

[In the days of Elijah the people suffered (an actual, non-symbolic) three and a half years of famine (Luke 4:25, James 5:17).]

In all of these cases trying to turn these periods from symbolic years or days to literal years or days is inadmissible. If I divide 1260 apples for 360 people I will never get a certain number of oranges. It is misleading to call these "prophetic years", it is probably better to call them "symbolic years", or "symbolic days": the length of time cannot be calculated, all we know is that they will be times of trouble, persecution and suffering.

It is noteworthy that 1290 days is one month longer than the symbolic three and a half year period of 1260 days, and 1335 days is 75 days or two and a half months longer. I think it follows that these numbers are symbolic days only... it is inadmissible to try to turn these periods - 1260, 1290 and 1335 days - into actual lengths of time.

It also suggests that the week in Daniel 9:27 consisting of two periods of three and a half days is a symbolic week.. it is inadmissible to try to turn this week into a specific length of time. Rather, it is identifying a period of intense persecution, double trouble, for the people of God. This would fit neatly with the view that the final 70th week in Daniel 9:25-27 is the entire Gospel Age, starting with the Word of the Lord coming to John the Baptist (Luke 3:1-3) and ending with the Second Coming of our Lord. [In Daniel 9:24 the 70th week is part of an actual length of years, but not in Daniel 9:25-27.]

The coming of the Kingdom of God, far from being a period of great dominion of God's people, in its first phase up to the Second Coming of our Lord, will be a period of persecution, sometimes intense, for God's people.

Blessed is he that waits and comes to the thousand three hundred and five and thirty days (Daniel 12:12).