The Borg do not evolve.
Strictly speaking.
The Borg is not a species per say, it's just one dude.
How that dude (the entire Borg Collective) "changes" over the course of it's life story, is from adding biological and technological distinctiveness to it's character, the hivemind, rather than how evolution works, where inferior aspects (stupid children who can't breed quickly) submit in favour of superior aspects (fuck happy mass-breeders who get a lot and don't use protection.) allowing for the heritage of the mass breeders to be passed on to another generation and the unf###able to be forgotten, which is in one sense random and in another totally/reliably junks the undesirables.
There is an unpredictable correlation between changes to the Borg's character from assimilating a new species, and solving problems facing the Borg that need solving, and accessing the good vs bad that came from achieving some sense of victory. The Borg may chose to assimilate one species because it has obvious desirable, and maybe even necessary qualities, but what about all the sediment and scum? The Borg must be constantly having to stomach a mass of undesirable skills and notions that they were not expecting just because they... Assimilating a species is like buying a second hand car from a dodgy car yard full of lemons.
In theory the Collective fundamentally changes every time it adds billions of souls called a species/civilization to it's think space, but there are already so many voices in there in consensus, trillion-trillions, that adding a billion voices of dissent at a time does not put a chink in their resolve, before any new billion people added to the Collective are swayed to the party-line.
Semantics aside, it's possible that Borg can't change how they think through assimilation, or evolve how they think via trial and error.