My GP build is arm pen reds 3 crit chance quints crit dmg yellow and crit chance blues. Why is that at the beginning of the game the crit is awesome but as it goes on the chances are less and less. Note: I'm stack crit items and arm pen items.

hello

The chances are less and less because the game is telling you that it sucks, that you should stop using it and get a real runepage/build

It could be due to your Crit doesn't have enough Attack Damage to back it up. Also remember to build IE whenever possible.

You do need armor pen and attack damage to do enough damage, so though you can get lucky and get a few 250 parleys at level 1 and with the level 2 gank pretty much guaruntees first blood with a little luck, it falls behind simply because you don't have enough damage, you can't expect to build all phantom dancers and get output from it. You can negate it a little bit by buying Yommu's ghostblade for some extra armor pen and damage and more stats, or a last whisper which will be your main source of armor pen, either one works really, but I like ghostblade for extra crit, and flat amount of armor pen and useful active. I would however, consider using a more consistent damage page like AD quints, with pen reds, and with MR/level or flat MR blues depending on your opponent and flat armor yellows or crit yellows depending on your opponent.

Never depends on crits no matter what champ you're using, its always a risk. Instead use more defensive runes such as hp and armor. It pays off a lot better in the long run and you wont find yourslf praying for crits anymore xD