It's called politics, @edpc . The passage of this bill is not important enough to stop Congress from making a point about more important legislation.
The farm bill is very important legislation. Not just because of the stability it provides to farmers - currently facing low prices and hard times - but because (through an historical quirk) it is the means of funding SNAP, one of the biggest social welfare programs.
Getting a handle on runaway entitlement programs has traditionally been far more of a conservative concern than appeasing nativists. If the FC had joined with other Republicans, work requirements could have been imposed on SNAP recipients. That could have emphasized the welfare-to-work culture while at least starting to rein in deficits caused by out-of-control entitlements.
Instead, the FC has doomed these efforts, in favor of pushing their anti-immigrant pogrom that has the potential to nip economic growth in the bud. Legal immigration is necessary to our continued prosperity; statistics released just last week show American birthrates at their lowest rate ever. American companies will soon be cancelling domestic expansion plans if Know-Nothings get their way and make it impossible for companies to predict how they will satisfy their future employment needs.