Here is a simple question: Why is there only one "Tor". Why haven't there been more onion routing projects. (Maybe there have been and I am just not aware.) Perhaps the same reason(s) we never saw widespread adoption of remote proxies, despite their usefulness in many situations. Although in some respects onion routing seems quite an improvement over "simple" proxies.
The more nodes you have participating the more secure an onion system tends to be. Since the Tor network can carry most kinds of traffic, the motivation to avoid a fork is strong.
Tor isn't very large as it is, and (I would guess) it's the largest. If another onion routing network didn't grow the audience, you would have two even smaller networks.
> the Tor network can carry most kinds of traffic
Isn't Tor limited to routing TCP? That would rule out QUIC, for example.