Anti-competitive behavior is why. Apple had no need to update their unknown tracker alert software on any particular timescale. And if Google was willing to wait until Apple made that update, Apple could certainly drag their feet and delay Google's launch for a while.

Presumably Google would have, eventually, said "ok, enough is enough, we're releasing on X date whether or not you've updated yet". And maybe that's indeed what happened! At that point Apple really would want to finish that update, since not doing so could be a privacy hole for their customers. (The same privacy hole Android users had to deal with in the early days of AirTags when we couldn't detect them.)