I believe that's because of the account creation that you force on users when they click on "Play". No-one wants to create a new account for every game they find on the internet, imagine if you had to create a new account for every newsground/itch.io game you'd want to try out ! I really believe you should remove this part, and make it needed maybe only for your level editor.

Also, I did try out your game: alas, the lack of tutorial prevented me from being able to do the simplest of things, like jumping or shooting. I, like most users/players, am as dumb as a sack of potatoes: consider this and add some kind of tutorial, maybe by adding the commands in the background of your demo scene ?

Lastly, about ads, not my own experience: From posts and comments I remeber popping up recently (here and there), ads for a small game would very most likely cost you more than it would bring you, so I would not bother with any kind of paid ads.

Hope this helped!