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??
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.