Starcraft's popularity was the result of lack of decent competition in what was at the time a niche of online games as a whole not just RTS games, it was maybe not first to market but it did benefit from a less competitive market with fewer real alternatives (Quake? Red Alert? I have no clue what else people played 1v1 online on a large scale back when SC was released). You played what was available on the market (or in the internet cafe) and the chance of getting into the game "by accident" due to the lack of alternatives was high. You walked into an internet cafe and you saw it and became familiar with a game, like it or not. The rest was snowball effect.
I won't say that is entirely wrong, but a much bigger factor that old blizzard was instinctively aware of and most others fail at is offering a fucking 'Play' button. You click it and bam, you're in a game against another player about on your level of ability.
You may say 'but raw why does such a small button make such a big difference you're crazy'. Several reasons.
1) Technical excellence. Having a play button also means you have a network infrastructure behind that button where games can actually happen on. This is no small feat. There is a reason old blizzard massively invested into battle.net. They instinctively understood the importance of the then emerging broadband internet and
deliberately decided that they wanted to make games on the edge of what is possible and claim technical dominion. And they did.
2) Competition. A play button allows for rapid games in succession against players all over the world. We take this for granted now but
this has only become possible with the advent of the internet. This is basically the extension of the first point: Blizzard realized that by having the technology in place you could do things never done before and design a game in an
inherently competitive way, that is, just like we have designed all the offline games for thousands of years. They understood that by underpinning the play button with a chat & ladder system you add replay value and foster a community.
3) Humans before machines. All classical Blizzard games are deliberately designed to be consumed by humans, not by machines. It is important to understand that. In any game of Starcraft there is just the units of you and your opponent and the board. Nothing else.
You play the game. Within the rules and boundaries set out by the game developer.
This is why World of Warcraft was a betrayal of literally mountain shattering proportions: Not only did WoW directly betray the principle of humans before machines. Suddenly you weren't playing with and against humans but against machines, puppeteered exactly in the way the game developer wanted them to. You die when the developer wishes so. You deal exactly as much damage as the developer wants you to deal with a weapon that you found where the developer wished you to find it against an NPC that is exactly in the place where the developer wants it to be with exactly the amount of health the developer wants it to have. You were made a slave to the machine. But the betrayal of Blizzard went even far beyond that: It completely shattered the eons old mountain of thought that humans play games with each other and not with against the machine. By violating this core principle, Blizzard, with their dominating position inside the industry back then ushered in an entire eon of "games" where you are made slave to the machine and it lasts to this day.
After the Faustian pact Blizzard made by giving birth to WoW their demise was sealed and the remaining core values (and engineers) were quickly thrown over board. Sure, Blizzard wasn't the only Company that has 'lost it'. But they are exemplaric as their fall from grace was truly the highest. And where they landed there was nothing but hell.