There are a few reasons for that:
- Google Plus demonstrated Google cannot build a social network
- Steemit would benefit from Google background technologies (Translate, Maps, AI...) and infrastracture
- Google doesn't have a cryptocurrency yet
- The branding would raise the Steemit value 10 times alone
- Compared with Facebook, Google would be the good company again (by sharing the value with the users)
- Steemit could be later integrated in Chrome