It's super difficult to build a low latency mix-net that covertly works inside an adversarial network.

In addition to the current models over which onion/garlic routing are based upon you would (at least) need to add to the core of your software traffic obfuscation, a series of covert channels, NAT bypass, ...

Tor and other mix networks simplify the problem assuming that there's a portion of the Internet that is free and introducing censorship circumvention mechanisms.

Of course, the assumption is increasingly untrue.