I'm not sure they'd be such a useful aquisition to the big players, as their strong point is the usability of their app itself, rather than the underlying technology so much. Citymapper already use Google Maps for walking directions and actual mapping, and Google Maps already have the same transit data that Citymapper gets from public APIs.

Citymapper takes a very different approach to UX to Google Maps so I can't see them going together well. Perhaps it could be used to improve Apple Maps, but again I think it would clash with Apple's usual way of designing apps in house.

Google did buy Waze, so they are open to maps acquisitions.

They bought Waze "mostly" for the userbase, but the fact that they have some of the best traffic data is useful too.

. . . and is interesting as it's almost completely orthogonal to this.

Google's existing offerings fail most spectacularly for knowledgeable people in urban environments (I cannot overstate how much GM's handling of subways drives me insane), which is the niche this lot have successfully found. Apple seem to have a lot of customers in that bracket so they are the better fit.