Are they really forcing us to use the app for maximizing user engagement? I tend to think they can syphon more user data from an app than from a browser and that would be their motivation.
While I have my tinfoil hat on, let's continue. I think reddit gradually degraded the mobile browser experience on purpose to push users toward the app. Seriously, how hard is it to show an image without clipping it? Images were fully shown before, then you had to click on individual posts to see them without cropping. Forcing people to expand comments is another one. Before we had infinite scroll, then you had to click many times to see the full extent of comments... And then a bit later, passed a few levels of nesting you were forced to use the app.
I don't know if an app like SlimSocial (which let you use Facebook's mobile site with the embedded messenger) exists for Reddit. I hate being forced to accept "permissons" to install / use an app. Personal data is the price to pay for services nowadays.