Because giving valve 30% of a new sale is better than no sale. Again - the only people buying the game on steam are people that are “no steam no buy” or people buying the game again. Diablo doesn’t need exposure, it’s one of the biggest gaming franchises around. What they want is more players and more money - not because it failed, but because that’s what profit driven companies do.
Also microsoft release all their PC games on steam, and Microsoft just bought Blizzard so this was likely done in preparation for the purchase.
What about it? You’re surely not going to try and use the literal best selling game of all time as an example of something are you?