The obvious answer is "because you've watched enough of them that the algorithm thinks you want more," but beyond that Mario games hit a particular sweet spot of being popular enough that lots of people have played them (which in turn means that there's a large audience of people interested in seeing them played differently), as well as having a skill gradient that's really obvious to anyone playing (that is, playing normally, you can see ways that a speed/challenge runner or other skilled player might cut corners and play better than you currently are) that makes speed/challenge running very accessible. The larger player base also means the speed/challenge running community is particularly large, which means lots of competition and lots of people making discoveries and contributing to researching new techniques, both of which help to drive activity within the community, which in turn means a relatively large number of videos showcasing that activity. Even for people that aren't actually interested in doing speed/challenge runs themselves, the popularity and accessibility mean that lots of people can say "I know why completing the game without pressing A is hard, so I'm curious how somebody else pulled it off," and that drives views and engagement with videos showcasing such runs.
In particular, Mario 64 has been dissected down to the literal source code by its speed/challenge running community (pannenkoek in particular offers a lot of ridiculously in-depth explanations of this, like his recent 4-hour video outlining all of the game's invisible walls and why they exist). While other games in the series haven't seen quite the same level of scrutiny, the precedent set by the Mario 64 community does inspire similar efforts from other Mario games' communities.
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