Even though Angular2 was released in 2016, there wasn't really an easy-to-follow upgrade path until mid 2018. (ngMigration-Assistant wasn't even released until summer 2018, and people didn't really start releasing in-depth tutorials until around this time.[1])
Since React was released in 2013, it was already pretty popular by 2015 when Angular2 was announced. So a ton of startups migrated from Angular to React in between 2016 and 2019, when it wasn't clear there would ever be a good upgrade path, or that Angular2 would actually develop into a good product.
Now obviously in retrospect there is now a very good upgrade path, and Angular has actually become quite good, but it wasn't at all clear that either of these things would happen for several years. But yeah, I'm sure there is a time effect here, but I think there might also be something more nuanced going on. Like no doubt most of these startups that are selling are 5+ years old, but that still puts us well into the time when React was popular.
[1] E.g.: https://www.youtube.com/watch?v=qeQljCLpR04, https://www.upgradingangularjs.com/