The US held off, mainly because their domestic suppliers Pfizer and Moderna already provided ample #'s of mRNA vaxxes and AZ was not needed. Pf and Mo are US produced vaxxes. AZ is UK produced.
The side effect happens in 1 case out of 60,000. That shouldn't bother anyone trying to avoid COVID. Those chances are way better than your chances if you get C-19.
Now that Canada has given 1 shot to a significant % of people and has increased availability of Pfizer and Moderna, there is less need to use AZ.