Devs cannot just use a different platform without cutting out a huge userbase.
Most gamers I know, don't care that much about where they get their games from. Heck, they would buy it from a dubious guy with USB sticks who is travelling on a donkey, if they had to.
Make a good game and people will follow.
I suppose the issue is rather about discoverability and getting people to learn about a game they might like. But steam is not necessarily easier in that regard. You'd still need to do some marketing to get the word around. If you're lucky by scratching the itch of a lot of people, steam can boost sales by its trending and discovery lists, sales highlights, etc. But that's not easy to achieve just by choosing steam as a distribution platform.
I wish I could. We would have so many cool children. /j