Why did email use develop in tandem with the internet, rather than other technologies?

I was using email on a few different systems long before I had an internet connection.

Email usage in offices depend on having a network and most people having a computer on their desks, this did not happen until the 1990s to any great extent.

Connecting computers from homes into email systems and cheaply connecting networks together depending on modems, it was not until the late 1980s that modems become reasonably priced.

In the UK universities were all connected to an X25 network that was separate from the internet, but had email running over it. There were a few gateways that allowed emails to be sent to people on other networks.

Companies subscribed to services that provided servers all over the world that there sale staff could use to send emails to head office with a local phone calls – these services were not connected to the internet or each other for a long time.

In about 1995 I could arrange the PA rota for Church (Cambridge UK) over email, none of the people I emailed had internet access, it was all done using different gateways between email systems. Within 6 months internet connection had come down in price in Cambridge so much that companies started to connect there lans to the internet and all my emailing could be done over the internet.

I think it was just that the timings of the changes that enabled wide spread usage of email happened at about the same time as wide spread usage of the internet become possible.

(All of this was before web browsers could display images!)

How does it come about that one technology dominates?

There are many ways that one technology dominates.

• Being "better" than competing approaches.
  Consider a house. When there was only the option of sleeping in a tree, on the ground, or in a cave, a primitive house offered many advantages. Today, a high proportion of the population live in houses, but other technologies are deemed "better" in many cases: units, apartments, and high-rise apartments.
  Consider bronze swords. Swords essentially developed from bronze daggers, and before that from stone knives. In comparison, bronze is less brittle than stone such as obsidian or flint, so a bronze dagger has an advantage (so say nothing of the associated prestige). A bronze sword provides more cutting "power" than a dagger, and allows one to outreach a dagger-armed opponent, so the sword has an advantage. But one does not replace the other - both have their uses (ie dominate) in particular circumstances. But then iron and later steel weapons dominated the bronze. Bronze can produce a very sharp edge, but in comparison to iron and steel, bronze is brittle. Iron was soft and soon steel came to dominate - not as sharp as bronze, but far less brittle; sharper and harder than iron.

• Being the first.
  Being first can give a technology a huge advantage. Trains could be considered an example of this, if you accept a definition like "the first mass long distance transportation". People could and did travel across whole continents on foot, horseback, wagon or coach. But the advent of the train enabled mass, relatively fast travel at comparatively cheap prices. If you were near a railway station you had fast access to the rest of the world. It took two centuries for competing technologies, such as automobiles and aircraft, to be invented and then bridge the gap.

• Through marketing, accident ...
  Not every "superior" technology dominates. The water is murkier here - did VHS dominate Betamax through superior technology, accidents of stubborn corporate decisions, or marketing?

In the case of email, it was a technology that was a good fit for the nascent internet: it was designed specifically for the purpose (see RFC821 and RFC822) and fulfilled a need. Before computers were permanently connected as they are today, email could be sent in hops from one machine to the next when they "dialed in" (often on a schedule). As connectivity has grown, so email has come into it's own.

The forerunner of the Internet, ARPANET, was managed by ARPA. ARPA determined how their network operated in a rather engineering-like way. A working group would be assembled, and would discuss and decide on what they thought was the best approach to a problem. The working group (WG) would then issue an RFC, which unless modified became the standard approach. The same process continues (see Wikipedia and The Internet).

So on the internet, there are no directly competing technologies. In a sense, email is the first, and the best at what it does.

The great advantage of email over telex or fax messaging is that email is "logically" addressed to a person, wherever they may physically be, and is (supposed to be) private to them. Whereas telex and fax messages are delivered to a physical location, and is not intrinsically private.

In the 1980s most people did not have computers, except the college-educated upper middle class, and even then it was mostly knowledge workers, young people and gadget buffs who had them. Nobody had telex machines. Fax machines were only found in offices and there might be one fax machine for fifty people. I worked at one architectural firm, the type of place where a fax machine was critical, and they had one machine and over 100 employees.

In the 1980s, the main method of computer communication was via bulletin boards, called "BBSs" (bulletin board system). These did have "email" in the sense that you could leave messages for other people if they joined the same BBS, which was free and easy to do. Thus, there was email in the 1980s pretty readily available for anybody with a modem board in their computer, which was fairly common. All the computers we had growing up had modem boards.

Note that the modem uses the phone line when in use, so if anybody called the house while you were on the BBS, they got a busy signal. Also, if mom picks up the phone and hears the modem noise, you get yelled at, "GET OFF THE COMPUTER!"

The key advantage of the internet was to allow computers to connect through a second line and the universality of participants. In the 1980s you just had a few geeks using BBSs (though technically anybody could do it). The web, especially AOL, induced a lot of "average" people and companies to join the internet. This mass of people made email much more viable.

AOL, by the way, was kind of a transition between the 80s and the full internet. AOL was like a super-BBS with millions of users, so it was easy to message a large number of people on AOL by dialing up. Of course, once the internet became widely available dialing into AOL was redundant.

Your basic premise is incorrect. Email did not develop in tandem with the Internet; the Internet simply made it available to the masses.

The use of "electronic mail" actually predates the Internet by a considerable margin. Electronic mail was used on ARPANET as far back as the 70's. The first standards were were proposed as early as 1973 (RFC 561). Throughout the 80's and beyond, email was widely used by folks who dialed into electronic bulletin board systems (BBSes) and other store-and-forward networks using phone lines and telnet connections. The email sent in the early 1970s looked quite similar to a basic text message sent on the Internet today, but the adoption of email simply grow in tandem with the use of electronic communications in general.