We'll never know for sure, but my guess is that Russia misjudged. They expected a combination of:

1) A massive a show of force,

2) A "Russia-friendly" Ukraine; and

3) A war-adverse Ukraine leadership

would collapse Ukraine quickly. We saw how that turned out!

Russia continued to ramp up (1) gradually while avoding heavy civilian casualties, learned that (2) only exists in certain areas (now annexed), and (3) doesn't apply when the US will provide unlimited billions in cash and equipment. That was March-August.

It wasn't until Ukraine scored some political and tactical victories in September that Russia said, "OK. Gloves off now." And here we are.