Why don't more people buy PC games from sources other than Steam?

some people can't buy from amazon US legally (even if seems legit)

thing is many dont know which ones to buy from i introduced 5 of my friends to gmg and now they buy mostly from there

I'm cool with Amazon, but when I try to purchase from GMG my cards and Paypal all decline on me. Gamersgate definitely has had a few too many issues for me to buy with confidence from them.

I use origin and battle.net quite a bit. Almost as much as I use steam.

A lot of people just start-up Steam when they want to play games. They don't usually browse different sites for a possibility to buy Steam redeemable games. They either don't know they can or they just don't bother. Many deals on Amazon run "only" for couple of days. Design of Amazon store for DD is not the best one, forums are filled with post "does this game activate on Steam?" and then those credits; even now on CAG forum people are still asking what is it, where can I get one - and those are people that know about Amazon and forums related to its sales.
Many of my friends play 2-5 hours a week. They don't have time to go to some forum to find out whether the game is Steam redeemable, where to get coupons/credits to save few bucks. They buy the game on Steam (-50%/-75%) and they are happy.

No PayPal / credit card. I buy my games with PaySafe cards.

I think lots of people do, but I have heard several reasons why some people don't, chief among them being that they like all their games/info on one application or site to avoid having multiple accounts across the internet. I personally have accounts at probably every PC digital service that often has deals and it doesn't bother me - some people just work differently, I guess.

As an aside, I've also heard some have issues purchasing from sites like GMG or GG for various reasons, so there's that, too.

Becouse Steam use dirty tricks

As far as I know, the Steam Wallet funds are non-transferable. So, since the Steam Market came out, I'm more inclined than ever to buy directly from Steam just to use the wallet funds I've earned through the market. Nothing against other sites, I just don't see a point in buying there until I've depleted my Steam Wallet funds.

Because Amazon isn't available in alot of regions. I want to buy from there, but I don't live in US and it doesn't accept paypal. Can't buy Mark of the Ninja there at the moment, even though that deal looks great, they won't let me. And Steam used to be the cheapest by far until recent years..

for that price you could have gotten MP3 AND the season pass lol. The 66% discount on amazon is a joke.

They do

Even if you use a credit card outside of Amazons market (Australia is) you cant buy direct download games. You can find someone to help though. I had a friend buy some games and send me the keys to add to steam library. Works fine that way.