They are BY FAR the largest producer of CPUs for laptops, desktops, and servers. Note that on each of these platforms, arbitrary code execution is an issue.
Now for phones? Less so. Aggressively locked down software can help.
So, as a researcher, who are you going to research? AMD, who has negligible market share? Apple, who completely locks down their platform? Qualcomm? Well, that's an option, but Intel still makes them look small. Vulnerabilities in Intel CPUs affect the most people and the most money... You're naturally going to put more research into Intel.
It absolutely does have something to do with how large Intel is.
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.