The reason there aren't any ads on pc is because it the game cost money. They make more money on mobile based on that they earn concurrent money for everyone that views an ad, and people can pay to remove them, and after every game there is an ad. So lets say that they earn 50 cents for every viewed ad and there are 5,000 mobile players on at one time, this means that if they all only play 5 games they will make $12,500 and everyone knows that they don't only play 5 games and let me remind you that this is in one day! They said that there was 110,000 people on at one time and most of them are on pc, that means that they have made around 500,000 dollars on just those people online at ONE TIME, and that is only if there are 100,000 people in total and that isn't true because there are different time zones and multiple countries so my humble guess is that they have made around 1 million dollars (give or take) and this is why they want a sequel because this game can take off.
You're naive if you think that they make 100% profit, penny for penny. You're also naive $1M is a lot of money, especially when it comes to running and maintaining a game on this scale.
Ads pay per click, and only 1.5% of people will click on a banner ad. Per 50,000 ads displayed in one day, only 750 will bring revenue, which ,per click, is closer to $0.10. That's $75 per 50k ads.
On top of that, server rental costs, additional fees for hardware maintenance per every physical server unit rented, both billed annually. Then they pay their business costs, then they pay their publishers, then they pay their taxes. Whatever's left goes to the three programmers who spent three years on the game.
They want to make a sequel because they didn't expect the game to blow up like this and want to implement as many failsafes as they to make sure that the game is as enjoyable as possible for the player base. Indy game devs don't make games cause it's profitable, because it sure as hell isn't, and if they were looking to make hella profit, you sure as hell wouldn't find on itch.io.