UART merely gives you a stream of bits - easy to decode - but USB is not just very much faster (hundreds of Mbit/s) but adds huge layers of protocol on top.

I recently saw a reference to an embedded USB protocol stack which described its 133K memory footprint as "small"

Clearly, developing an analyser for such a protocol is a major project all on its own!

And, of course, the whole point of having an analyser (and tester) when you're developing comms stuff is to test it - if you use your own protocol analyser to test your own protocol implementation, what confidence does that give you?!