Ranboo loved Tubbo, and Tubbo loved Ranboo. PLatonicly of course, it was undeniable.
Tubbo and Ranboo also loved Tommy. But Tommy didn't feel as loved, feeling like he was the third wheel to their relationship. He wasn't as loved. Left behind as he died in prison.
That wouldn't do.
Besides, Tubbo and Ranboo had enough love in their relationship to add a third.