Because now they have to report it to their shareholders where the money is going so that if the board doesn't like it they can replace the CEO. Before since it was all google, the money went where they said it went, there was no oversight. They had this massive R&D budget that was opaque to the investors. Money that could have been paid to shareholders as a dividend or return was instead spent on projects they had no idea about.
Alphabet is still spending billions from Google into "other bets" like Loon, so I don't see how this explains the change.