Some of these novel side-channel attacks actually even apply in completely unrelated architectures such as ARM [1] or RISC-V [2].
I think the problem is not (just) a lack of competition (although you're right that the duopoly in desktop/laptop/non-cloud servers for x86 brings its own serious issues, I've written and ranted more often than I can count [3]), it rather is that modern CPUs and SoCs have simply become so utterly complex and loaded with decades worth of backwards-compatibility baggage that it is impossible for any single human, even a small team of the best experts you can bring together, to fully grasp every tiny bit of them.
[1] https://www.zdnet.com/article/arm-cpus-impacted-by-rare-side...
[2] https://www.sciencedirect.com/science/article/pii/S004579062...
[3] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...