Instead of being amazed that TikTok beat juggernauts, maybe it should be reframed that they beat companies that have terrible products.
Sine we're talking about niches and customized feeds, let's look at reddit. My god is the reddit app absolutely horrendous. It's so bad I use the web app in desktop mode because other forms have terrible UX. They just don't care.
Or let's talk about another site that was mentioned: youtube. It's like Google with all their money, doesn't really care to spend real money on trying to solve problems. If they dumped something like 100-200 million into better community outreach, and actual going in and tagging videos, and dealing with content, the service would be so much better. Instead, they made their money on search, and now it's spinning it's wheels trying to find clever algorithms to solve problems.
We knew there was a market for an app that lets you easily create, edit and upload videos. Vine proved the formula worked. TikTok stepped in and filled the void.