that is a really good question. One nuance to your answer might be found in this video.
Basically, Ben Bernanke once defended the Fed's QE Program by saying, "What we did was Qualitative Easing, not Quantitative Easing." The Fed made a specific effort to buy securities like MBS after 2008. In a true QE Regime, the Fed would buy comic books if they could - they just want to pump money into the system.
Your Q is good, and one answer I can come up with: the Fed might want to buy specific ABS like those packaging restaurant franchisee receivables. This is obviously because the food industry has fallen off a cliff - as have any securities associated with that industry. So, maybe
$1 spent buying those ABS is better than $1 of MBS, etc.