It's all about better tracking. I'm not quite sure what additional info they get exactly, but tons and tons of mobile websites (that work and don't get deleted) are close to unusable due to a barrage of popups telling you to use the app (e.g. Reddit and other socials).
Also there is no indication they will stop the mobile web version. Already today the mobile web version is there but it explicitly refuses to show the boarding pass QR code: https://i.redd.it/lj3wdnfp9mq91.jpg
But they refuse to do so in order to get all that data which they can sell. In a mobile app it's way harder to run ad blockers and much easier to sneakily collect information on the user. Especially on android which is by far the biggest OS in the countries where Ryanair operates.
I'd think it's only maybe 5-10% of customers at most who both use desktop over mobile to get their boarding pass and use an ad-blocker on desktop. And honestly I don't remember ever seeing an ad (even on Ryanair) when getting my boarding pass on mobile. OTOH I distinctly remember seeing many giant ads on printed boarding passes, most often on printed boarding passes brandished by other customers (usually printed in full color!). I'd think that's hugely more valuable as advertising real estate than the iota of additional data they get to collect on a few adblock users who have been forced to use mobile.
And even without adblockers a mobile app can gather much more data on you than a website can.
probably more guaranteed location tracking - hey this guy is buying tickets from the expensive part of town on the newest model iPhone! Chance we can jack up the price, 99% good!