According to mainline Protestant theology, why is Pentecost on Sunday and why did it lose its significance?

SLM has given a full explanation of how Pentecost must be on a Sunday. All I would add is that the season of Shavuot, identified in Acts chapter 2 as Pentecost, is bound up with Passover. The two holidays are merged by counting seven full weeks, plus one day (50 days), from the second day of Passover.

Just before Pentecost on Sunday 31 May this year, I received a copy of the booklet Beshorah! published by a Messianic outreach based in England, and the main article was all about Shavout and 'The Wave Sheaf'. The ancient history is fascinating. Here is an extract:

On the second day of Passover, a sheaf of barley was ceremonially cut down. Its grains totalled one ephah. The sheaf was brought into the Temple in Jerusalem and the grains carefully beaten off. They were 'parched' on a perforated pan so that the fire touched each grain. Next, the parched grain was winnowed. The remaining pure grain was then ground in a special mill and passed through a succession of 13 sieves. From the original ephah, only one omer (one-tenth of an ephah) of the fine flour was taken, mixed with oil and frankincense and then only a handful was burned on the altar. All of this is what was referred to as 'the Wave Sheaf.' 'Waving' describes the sifting that symbolises testing and examination by the Lord. This ceremony began the'Counting of the Omer'; the counting of the 7 full weeks plus one day, culminating in Shavout.

The Day of Pentecost: The 50th day after the Sabbath of Passover week (Leviticus 23:15-16), thus the first day of the week (Sunday). Pentecost is also called the Feast of Weeks (Deuteronomy 16:10), the Feast of Harvest (Exodus 23:16) and the day of firstfruits (Numbers 28:26).

Season of Shavuot – Festival of Weeks or Harvest: 50 days from the second day of Passover on the 6th day of the third month Sivan (May-June). The 50th day after the Sabbath of Passover Week is the first day of the week = Sunday

I would also like to add that most Reformed Protestant churches in the United Kingdom consider Pentecost to be highly significant. The sermon on Pentecost Sunday was, not surprisingly, all about the outpouring of the Holy Spirit at Pentecost, after which Christianity expanded rapidly from Jerusalem into the furthest corners of Empire, and thence throughout the entire inhabited world.

EDIT: In response to your question asking why Christians do not celebrate Pentecost as the giving of the Torah, I found the following information in my New International Version Study Bible:

The Feast of Harvest is also called the Feast of Weeks because it was held seven weeks after the Feast of Unleavened Bread. It was celebrated on the sixth day of the third month (usually about mid-May to mid-June) during the wheat harvest. In later Judaism it came to commemorate the giving of the law on Mount Sinai, though there is no evidence of this significance in the Old Testament. In New Testament times it was called (the day of) Pentecost, which means “fifty”.

There is no biblical command for Christians to follow this tradition. The Old Testament law is a unit. Either all of it applies, or none of it applies. Either you perfectly keep the whole law or you fail (James 2:10). Jesus perfectly kept all of the law thereby fulfilling it. When Jesus died on the cross, He put an end to the Old Testament law:

For Christ is the end of the law for righteousness to everyone who believes. (Romans 10:4.)

Now before faith came, we were held captive under the law, imprisoned until the coming faith would be revealed. So then, the law was our guardian until Christ came, in order that we might be justified by faith. But now that faith has come, we are no longer under a guardian, for in Christ Jesus you are all sons of God, through faith” (Galatians 3:23–25.)

Speaking of Jesus who has broken down the dividing wall of hostility “by abolishing the law of commandments expressed in ordinances” (Ephesians 2:15).

In place of the Old Testament law, Christians are under the law of Christ (Galatians 6:2). Jesus has taken mankind’s sins upon himself (Galatians 1:4) as well as the curse of the law (Galatians 3:13). Paul is saying that the Galatian Christians are free from obeying Jewish ceremonial laws. However, that does not mean they are free from God’s moral requirements.

Jesus commands his followers to “love the Lord your God with all your heart and with all your soul and with all your mind… and to love your neighbour as yourself” (Matthew 22:37-39). If we obey those two commands, we will be fulfilling all that Christ requires of us: “All the Law and the Prophets hang on these two commandments” (Matthew 22:40).

The count to Pentecost involves a start and an end requirement in order to reach 7 weeks of 7 plus 1 day to equal 50 days. There's only one way to achieve that; start on a Sunday and end on a Sunday with 7 weeks of 7 between those two days.

THE START

And he shall wave the sheaf before the LORD, to be accepted for you: on the morrow after the sabbath the priest shall wave it. Lev. 23:11

THE COUNT

And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete: Lev. 23:15

THE END

Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD. Lev. 23:16

If one starts on any other day, one cannot achieve the start, count, and end requirements. The start is always after the weekly Sabbath, the count always requires 7 sabbaths, and then the day after the 7th sabbath or 49th day, which is always a Sunday.

So, this is the reason Pentecost always falls on the first day (aka Sunday).

SABBATH/7th DAY

Sabbath (Shabbat) is on the 7th day (aka Saturday) with day 1 (aka Sunday) per creation week (Gen. 2:2-3).

IMPORTANCE

A number of Christian denominations continue to honor Pentecost, including Catholic, Orthodox, Pentecostal, Lutheran, Evangelical, and many more.

PS To be sure, there are some groups who believe that Pentecost may fall on other days of the week besides Sunday, but anyone may do the count and find it must start and end on a Sunday with 7 weeks of Sabbaths plus 1 day to get 50 days.

In addition, THE END requirement details that the 50th day is the day after the Sabbath. That must be the referring to the weekly Sabbath and not to a feast Sabbath because there are no feast Sabbaths at that time, let alone on the 49th day. Thus that 49th day must needs be another weekly Sabbath.

OBSERVANCE

On the question about why Christians don't celebrate the giving of Torah on Pentecost, but rather focus on the giving of the Spirit on Pentecost, the answer is two fold about the old and the new.

The Law was given to Israel and it would be their righteousness to follow it.

And it [the testimonies, statutes, and judgments v20] shall be our righteousness, if we observe to do all these commandments before the LORD our God, as he hath commanded us. Deut. 6:25

Romans and Galatians and Hebrews contrast the Law and the Spirit in numerous ways.

That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit. Rom. 8:4

But if ye be led of the Spirit, ye are not under the law. Gal. 5:18

So, while Christians may understand that Torah was given at Mt. Sinai on Pentecost, we rather observe the giving of the Spirit to all who believe.

And when the day of Pentecost was fully come, they were all with one accord in one place.... And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance. Acts 2:1,4

Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons:... While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. Acts 10:34, 44