As a user of both (and investor in neither) it always seemed to me that Waze did the crowdsourcing thing pretty well but their map editing tools were really clunky, UI was much less responsive, and they just didn't have any good way to fund the service (as indicated by the fact that all I ever saw in terms of advertising were little Taco Bell icons any time I was near a Taco Bell).
Google has the backend, the framework for making money from the service, and established routing and mapping systems whereas Waze's more granular and up-to-date info on traffic conditions offered something Maps didn't have.
Basically, the reasons people used Waze over Maps also made good additions to Maps. Would've been fine the other way around too but the reality was that Google was the larger, more established company so in typical fashion, they bought the smaller one to gain their features.
(There was an accident on interstate 5 south, the backup was obviously affecting traffic on the highway but also the surface streets leading up to the onramp. GMaps had the most direct route, of following the traffic to the nearest ramp as the alternate route, with the default route being a snaking journey down some streets to the next ramp that was beyond the traffic.)