Let me explain my reasoning, and let me add first: this is just a hunch I have, I can’t back it up with numbers, so I might be wrong.
On mobile deivces, app stores are a normal thing. People are used to going to the app store, which is completely integrated, and they’re used to paying a bit of money for an app.
On desktop/in the browser, that’s not the case. The marketplace is fragmented, people have to sign up for different plattforms, enter their credit card info etc. So the threshold is MUCH higher. To avoid that, you usually will be forced to make quite a bit of content accessible for free.
Now if I have a game I can play level 1 for free on the web, I’m much less likely to buy the whole thing (whether on web or mobile/app ecosystem). It’s a casual game, so it might be totally OK for me to just play level 1. (which is actually completely what happens to most games which have a demo on the appstore. I’ll check out the demo, but usually won’t buy the game, unless I REALLY like it. So conversion rate from demo to full is pretty low, even in an app store environment.)
So that’s why I think they would have made less revenue even with a bigger (web-) userbase.