I suppose thats for multiple reasons. First would be that many of the slice girls are very solid stat, ability and skill wise. Some of them are ranked extremely high and do well in any team later on.
They also tend to have a good art design, considering they are ment to be fighters on the front lines, their desing tends to be more down to earth and relatable, so you are spared the suspension of belief (looking at you girl swinging a giant lolipop....).
There might also be other reasons based on personal preference per say.
Plus.... A beautifull women with swords.... that tends to jingle ones gemstones a little wont you agree ?

**On a side note - in DMM where the Critical hit feature is implemented, Slash type girls are the most prevelant ones who have it, there are few Pierce ones, even fewer Hit ones, and no magic type girl who has it. That also might factor into it, even if just a little.**