The only reason is that the status quo is they have in the past freely supported these use cases, but it doesn't seem that unreasonable for commercial use API access to cost money.
they haven't done so because they have chosen not to. they are still choosing not to.
this is a calculated move by reddit to extract the highest amount of money possible from 3rd party app developers, and the users of these apps are who is going to suffer. reddit waited until API use was counted on by some portion of its users before they pulled this lever. it's predatory.
I wouldn't personally pay for Reddit Premium so if ads are the only way to keep third party apps viable then so be it.