Or simply none of them were aware that the kinds of speculation they were doing were capable of producing exploitable vulnerabilities. It took security research

Or simply none of them were aware that the kinds of speculation they were doing were capable of producing exploitable vulnerabilities. It took security researchers a long time to find these issues, and the kind of speculation that the original Spectre and Meltdown exploited were well known outside of the chipmakers for over a decade. Intel was simply more aggressive at that particular avenue of performance improvement.