I am a person who went from being a Monotheistic Pentecostal to being a skeptical Deist to being a Polythiest. These changes obviously weren't sudden, but rather gradual shifts, however I still am often asked "If you can believe in many Gods, why can't you believe in just the one?" more specifically they are generally talking about the God of their faith (the Christian rendition generally) and I am writing this to clear up a few matters. Please note, I'm not attempting to "convert" anyone as I have not motive to, I'm merely talking about why I'M not Christian.
1) A God which is omnibenevolent, omniscient, omnipotent, and omnipresent is extremely hard for me to believe in whenever there is so much suffering in the world, even extending outside of a Christian perspective. Relegating it to merely a Christian perspective makes it even harder for me to do so, seeing as in the Old Testament he feels sorry for drowning everything, and has to correct the mistake he made with the whole sinful humanity thing. This sort of dissonance is not present for me in my current beliefs, however. Gods which are not omnibenevolent have no automatic obligations towards humanity, Gods which are not omniscient cannot be looked down upon for not having forsight, Gods which aren't all powerful cannot be chastised for the Universe not operating in a particular way. Omnipresence is generally a moot-point and is almost universally shared with all Gods (to some degree.)
2) The Christian God requires faith before experience, from a polytheistic viewpoint, my Gods do not require faith before experience, but rather for experience to generate faith. Now, this is not to say that I don't think that Christians have not felt "God's Love" as the refer to it, but I certainly never felt it in all my years of being a devout Christian. I saw others experiencing and I ached for the feeling of being touched by the Divine, I never was. I have been touched by various other Deities since I have taken a Polytheistic viewpoint of Deity, and this was crucial in generating Faith for me, actually feeling the presence of Deity. It would be horridly inaccurate to say that I believed in them before I prayed to them, I was merely open to their existence, and sure enough I discovered that being open to it was all it takes. I simply cannot believe without feeling first.
3) The Christian God dooms those who are not Christian. This is perhaps the crucial point of the whole thing for me, more than the dissonance that a Monotheistic viewpoint generates, more than the lack of ever having felt Yahweh's presence in my life, the Christian religion condemns those who are not Christian to Hell. That is all there is to it, (John 3:16-20, Acts 4:10-12), I cannot see a way for me to personally work around this, perhaps other people could, but I cannot. It is a pretty central tenet as far as I can tell. I simply cannot believe in such a thing.
There are a few other reasons as well, but these are the big three for me. Again, I reiterate, I'm not slamming against Christianity, I'm stating why I'm not Christian. I've explained this a few times, but I merely want to give a bit of a more thorough explanation than I usually can.