Because then they couldn't sell the same game twice because Platinum was just one version. Same for Emerald not being remade because that was just one version as well. Which is a terrible reason to not make the better version as the base, but money i guess.

HGSS could get away with having Crystal elements because all of its elements were divorced from the main story. Platinum and Emerald have enough differences in their plots compared to the first two versions of their respective generations that it would require story changes to justify two remake versions. And that takes more effort than they were willing to make.