|Not enough thread-lock ?|
The thieves must have had a trailer or a pickup truck, and probably a chest of tools too. The old cliche holds true: If they want it bad enough, they will find a way to take it -- you just want to extend the time it takes them so they figure they can't finish the job before someone sees them.
Also, the kind of characters that would steal an entire bike rack calls to mind the ATM theft scene from Breaking Bad. And that did not end well at all.