Playing a game in a browser requires less effort from users, so you are likely to have more people trying out the browser game.
As opposed to downloading a game, which requires downloading the files, extracting them, and find out where the executable is. Not to mention there’s more chances of something goes wrong, like the game not running properly on someone’s system.
I don’t think there’s anything you can do about this, it’s just how it works. The easier a game is to access the more people will give it a go.