The same proofreading machinery which lowers its mutation rate, though, allows recombination when multiple infection happens. I would argue that gives this coronavirus a mutational advantage over influenza making up some of the difference in base pair mutation rate:
The reason why we haven't seen a true escape mutation is more to do with the fact that you need mutations at up to 20 epitopes on the spike protein to collectively arise, and one mutation alone won't do it, which is likely to eventually be costly to the virus and it would prefer to maintain the same fitness levels and spread in the totally naive population (at the same time though the mutations in spike which come at a fitness advantage that have been seen are worrisome because it means the virus still has room to move on the spike protein without negatively impacting fitness). It won't encounter enough selection pressure to achieve escape mutations until you start to see >70% of the population vaxxed or recovered. Influenza itself can takes several years to mutate and come back which you can see by the waxing and waning of 2009 H1N1, which has not been epidemic every year, so we wouldn't have expected an escape mutation by now even on paper.