There are a few key differences that have allowed Covid to spread much more readily then does influenza.
1) There is zero background immunity, as compared to influenza where there will be since it's been around forever.
2) Longer period for incubation for Covid prior to symptoms.
With influenza you tend to develop symptoms rapidly, and also don't really actively shed the virus until you are displaying symptoms. If you are symptomatic, you are more likely to self isolate (especially with all the Covid protocols in place) whereas with Covid, you can be asymptomatic for 2 weeks, shedding virus all along the way during the asymptomatic timeframe.
What makes Covid so insidious to me is that you can have a very healthy dude like Khazmat really laid out by the virus, while a fat POS like Donnie Two Scoops gets through unscathed. I've seen patients > 100 years old come through just fine, and apparently healthy folks end up in the ICU.
It is a strange bird.