Honestly, In don't know why your game is getting so low a score (I mena, you have a good score, but it SHOULD be above a 4) ....I mean, it has a few flaws here and there, but it's stuff SO much fun!
Here are the only things that I would improve:
1. Infinite levels (just set some kind of automatic genrator thing)
2. a reload speed upgrade, because I felt like rate of fire upgrades were kiond of annoying after the first upgrade, because when you spend your ammo faster, you just have to reload at the same horribly slow speed!
3. A few powerups here and there (like, a meter where you can ask god to clean the current field for you when it fills up, or mabye it costs a lot of money, or something like that)
But ya, besides these things alone, this game was darn near PERFECT!