Why didn't Congress pass a budget last year?

I hear a lot of complaining that Congress should have passed a budget last year, but didn’t make it a priority. Now it’s the Democrats’ own fault for not getting it done when they had the chance. I also read somewhere that one reason they couldn’t pass a budget last year was because of filibusters, but budget votes can’t be filibustered.

Is there a plain and simple answer based on fact? Links to back it up would be highly appreciated.