And that explains that ARM, Sparc and other processor architectures are also affected, how exactly?
It is an industry wide problem. It is something that dates back to the 90s, when processors weren't used for virtualization and weren't connected to the Internet. The processor designers had taken a line for designing performant multi-threading processors, then the industry decided virtualization was a thing and that connecting to the Internet was a thing.
Instead of going back to basics (and temporarily crippling the performance of new processor generations), they built out the current architectures (PowerPC, ARM, Sparc, Intel, AMD etc.) to allow these new features, but without ensuring that such side channel attacks could be blocked.
Intel does have the most problems, as they have Meltdown as well as nearly all Spectre variants, whereas the other chip designers / producers only have certain Spectre variants to deal with, but none of them come up smelling of roses.