The market does decide, producer know that there is a big market for Halal meat mostly on the export side and it's simply more cost effective to run an halal certified operation that covers the combined market. to Produce for the two markets separately would increase costs to both, simple scale of economy.
Interesting that you make out that Kosher slaughter is more humane than Halal, the RSPCA have a different view. They think it's more humane to stun the animal first and a very much against Kosher slaughter, hence few slaughter houses are allowed that method.
Halal would much prefer to us the Kosher method but in a compromise with the RSPCA the animal is stunned into unconsciousness and the throat is cut. The RSPCA preferred method is that the stun actually kills the animal.
I would not argue which method is more humane but I dare say 50 or so years ago there was little consideration for this matter
