Because a Politician who panders to special interest is going to screw the voters on every other issue if it benefits them politically especially of they have a history of supporting liberal positions like being pro abortion pro national health care and many others.
This and attacking Cruz’s religion (I am a non denominational myself ) says a lot about Trump this is very negative IMO.
I would note he dropped his Muslim bomb the afternoon the poll show him behind Cruz came out. Panic move. If he loses Iowa and the veneer of invincibility is removed he could melt down, he is very narcissistic and thin skinned like Obama.
It’s a popularity contest among low information voters.