SDA was the primary place to submit speedruns, following the rather restrictive Twin Galaxies. It's still around and somewhat active, but due to newer websites (such as our very own Speedrun.com) that have proven to be more accessible to newcomers, SDA has become less prominent than it once was.
It's just the natural progression of things, not really due to one deciding factor that I'm aware of. The speedrunning community has never come to a singular website anyway, as a number of prominent games have their own speedrun websites, like Mario Kart and Mega Man.
It’s also worth noting that SDA tended to have stricter rules for games such as requiring all runs to be on the English version for some games. As the overall community grew and people both grew more creative with the categories they wanted to run, and found more creative ways to save time that didn’t fit in the more restrictive rules, people began to look for an alternative to SDA that would be more flexible.
