Bowser's main goal is to defeat Mario. He knows that if he kidnaps Princess Peach, Mario will do all he can to save her and might die in the process, 或者 at least he can lead Mario into a trap.
1. Bowser wants control of the 蘑菇 Kingdom and marrying 桃子 would give him the power to do it.
2. Bowser has a crush on her, which was first revealed in Super Mario RPG when he got jealous of 桃子 rewarding Mario with a 吻乐队(Kiss) instead of him. He's also secretly envious of the relationship that Mario and 桃子 has with one another.
3. 雏菊, 黛西 is a tomboy and is less prone to being kidnapped. However, she was kidnapped in Super Mario Land. But still, I don't see a reason why Bowser would WANT to go after Daisy.