I suspect these changes come from very different motivations. My guesses are ① Reddit needs money, and ② Twitter engineering has been brain-drained.
Reddit's changes are a desperate attempt to please investors. This is a standard tactic for tech startups that are having trouble showing a profit: figure out what the company has been giving out for free, and start charging for it. They looked at API traffic and said "what do we have to do to monetize this?" and the current situation followed directly from that.
This is not just a vague appeal to "the market"; there are specific big-money investors involved, and they will have been communicating directly with Reddit management on what they want to see from the platform. CEO Huffman is probably listening to specific advice (or demands!) from those investors, and making policy changes to appeal to them.
Twitter's situation is quite different. The goal of Musk's takeover was political: to reverse Twitter's user conduct policies, which had led to Donald Trump and other fascists being banned from the platform for shitty behavior.
Twitter is falling apart on a technological level right now because most of its skilled engineers have quit, leaving the company no longer capable of responding effectively to technical problems.