There are fewer people trying to attack AMD's chips, they comprise such a small chunk of the installed CPU base out there that most attacker's won't be going after a system containing a CPU from AMD. This may change in the next year or two though, at which point the vulns that are likely in their Platform Security Processor may bite them in the ass.

Qualcomm on the other hand has taken the "fuck security" approach and failed to clean up their code and get it mainlined for nearly all of their common SoCs. Hence why Android phones are often trapped on old versions of Android when launched or shortly thereafter, long term security through the device lifecycle is a bad joke when Qualcomm and most other ARM vendors are involved.