They didn't pass one because the Republicans filibustered it.
I would agree with you...but you're wrong. They had a filibuster-proof majority but still didn't pass a budget as is required by law.
You can't filibuster something that doesn't exist. In order for the GOP to filibuster a buget from the Democrats...the Democrats would first have to submit a budget. In case you haven't noticed...Democrats don't want to DO that because they think they'll pay a political price if they're forced to admit what their agenda costs the country.
The reality is...the GOP would LOVE to see the Democrats submit and VOTE on a budget because then they can use that bloated budget against them come election time. They're not going to filibuster it!