Fast.com has existed for 15 years yet isn't nearly as popular. It's easy to build a new speed test, but much harder to get people to use it.

Downdetector wins because of SEO. Most people don't get there directly, they google for "is $x down" and then get sent to downdecetor. Which from my understanding works by simply showing you how many people came to their site with those search terms. They don't actually check the sites.

Fast is a Netflix product so the fact that you've even heard of it is in direct relation to the weight of the brand that launched it.

speedtest.net has been the first search result on Google for "speed test" for decades. Partly the boost of domain SEO and partly the boost of it being an effective exit node for searches for that term for that long.

(Nobody searches "ookla" and nobody is going to search your tier-3 .com)