No one thing controls it. Masks help. Frequent hand washing helps. Social distancing helps. But, all it takes is one superspreader event to turn one case into 100+ cases.
Another contributing factor has been called lockdown fatigue. It's actually fatigue with all the precautions people are asked to take. This tends to result in lower compliance with control recommendations and a lot of "I can let this slip by, just this once" kind of thinking. This is one of the reasons why the 2nd wave during a rough flu outbreak is frequently worse than the first wave.
Add in the growing anti-science conspiracies encouraging people to ditch masks wherever possible and BS claims designed to frighten people away from being vaccinated, and containing the threat of this virus is going to take years instead of months.
P.S. Happy B-Day one day late Piglet!
It's been a sucky year for birthdays (and a lot of other things). I think we all need a double or triple strength party when things are better.