Erm, no? Meltdown was intel only. Spectre affects absolutely every architecture with speculative execution, but Meltdown (which allows crossing process and security boundaries) are absolutely unique to Intel.
Everyone who does speculative execution had Spectre issues, but Meltdown-style vulnerabilities have been mostly Intel-exclusive. These new ones are too.
Maybe because Intel has shipped a thousand more SKUs and millions more CPUs with Meltdown than ARM, for which the Cortex-A75 was a new design, and IBM, which doesn't ship huge numbers of either POWER or mainframe CPUs??
Why would that make a difference? We're not talking about manufacturing defects, every single unit they sell has the problem, doesn't matter if they sell 10 or 10 million.
It makes a difference because Intel is a more attractive and more consequential target for researchers. AMD's market share in servers is minuscule and even declining a bit as of 19Q1 (?), and modest but increasing nicely in notebooks and desktops https://news.ycombinator.com/item?id=19916279, while IBM's POWER and mainframe systems are expensive to very expensive to access.
ARM is actually a good target with a number of their newest designs using out-of-order speculative execution with Spectre vulnerabilities and their owning mobile space outside of notebooks, one of the newest even being vulnerable to Meltdown, but the significant headline worthy instances tend to come in much more locked down devices. Speed is also an issue, everything else being equal, the faster the chip, the faster data exfiltration proofs of concept will work.
Serious weasel wording, outside of x86 space every other high end architecture also had Meltdown issues, ARM, and IBM's POWER and mainframe designs.