Because Google routinely undermines their partners through internal productizing (see e.g. Flintstone/OnHub), and Google has a large self-driving car program. There's a clear path from there to a Google(/Alphabet)-owned ride provider regardless of investment in Uber.

Uber is building out their own map system so that they are no longer dependent upon a third party. It's already in production on a limited basis.

Isn't Google Ventures pretty isolated from Google?

Google has the map data. Google are working on autonomous vehicles. When they get the latter, what is the need for Uber? The $250M would be an irrelevant sunk cost at that point.