I'm trying to find out why a collection agency would wait so long (almost 2 years) to garnish my wages, after they already got a default judgement in court?

There's no way to know - might be a question of "hoping" you would pay without the necessity of a garnishment, maybe they took this long to find your employment, maybe something else.

Again, no way to know other than asking the judgment creditor.

The judgment remains open on your credit report so they have (probably) 10 years to move against you, renewable for another 10.