Why do browser implementations of HTTP/2 require TLS?
It is partly about making more things use HTTPS and encourage users and servers to go HTTPS. Both Firefox and Chrome developers have stated this to be generally good. For the sake of users and users' security and privacy.
It is also about broken "middle boxes" deployed on the Internet that assume TCP traffic over port 80 (that might look like HTTP/1.1) means HTTP/1.1 and then they will interfere in order to "improve" or filter the traffic in some way. Doing HTTP/2 clear text over such networks end up with a much worse success rate. Insisting on encryption makes those middle boxes never get the chance to mess up the traffic.
Further, there are a certain percentage of deployed HTTP/1.1 servers that will return an error response to an Upgrade: with an unknown protocol (such as "h2c", which is HTTP/2 in clear text) which also would complicate an implementation in a widely used browser. Doing the negotiation over HTTPS is much less error prone as "not supporting it" simply means switching down to the safe old HTTP/1.1 approach.