ECL logic systems reached effective clock frequencies in excess of 500 MHz in the late 60s or so. It was extremely fast compared to contemporary RTL/TTL logic.
MOS is just hard. It's hard to fabricate, it's hard to design (i.e. you need computers to make computers!), it's hard to scale. It required new chemistries be invented and new circuit design techniques to manage the (really tiny!) components. So for a long time you could have a big circuit in discrete bipolar components which ran fast or a tiny one in MOS which was slow.
But MOS was tiny and MOS was cheap. So by the early 90's it had caught up in speed with all but the fastest discrete technologies (GaAs was "right around the corner" for like two decades as it kept getting lapped by routine process shrinks -- now no one remembers it) and the market abandoned everything else.
I think the important point is that "MOS scales". All the bipolar technologies never had anything like Dennard scaling, which was the backbone of Moores Law.
They're topological circuit constructions that work for either family - aren't they?