Because right now she's still a minor. 5 year gap is ok if both are adults (I actually have a relationship with 8 year gap), but still a little bit icky if it's 16 and 21.
But overall I'm cool with SanPu, I want my boi Sanji to be on the receiving end of love often, and being the lady pleaser he is, it's quite possible for him to end up in a relationship where he gives too much and receives too little in return. I mean, Pudding certainly will have to work on her many issues, but she seems to be madly in love with Sanji and she will be as over the top expressing her love as he is.
Plus I like the concept of Sanji having his own Baratie EOS, and Pudding can help him out as a sous chef.