While it’s undeniably a more lucrative target, I still don’t think that is the explanation for the differences in vulnerability. Researchers have been working on the x86/amd64 family of devices for a very long time. Looking aside, many of the published exploits in the past have been against Via and AMD (older architectures).

Like others pointed out, the portability of an attack is usually tried shortly after a successful attack is found. In this case, the attacks have not been found to work elsewhere yet. I won’t count it out that it’s more effort but we’re looking at a timeline of research that spans a year after first reports were made to Intel, which is plenty of time to consider other chips. AMD’s specter problems are very real but much narrower while an entirely separate architecture like ARM shared a lot more of the attack surface with Intel, including Apple chips which you cite as locked down.

Your logic makes sense but the actual historical log of exploits I’ve seen does not seem to line up to explain the result. It only guarantees that researchers will try things against Intel chips first, but nothing about the exclusion of other chips.

Note: “Dwarves” is the Tolkien variant spelling of a mythical race of beings. The normal English word is “Dwarfs”.