I was disappointed to see Walker drop ,but I think that he was under tremendous pressure from the Establishment GOP. First he defended the indefensible John McCain and then there was something else that I can’t remember that led me to think that Walker was trying to play their game. I think he realized that it just wasn’t worth it, that he would lose himself if he kept trying to please the Establishment.
If I remember correctly it was Walkers stance on immigration that made people drop him. Probably a GOPe encouraged stance.
After initial leads in the polls, people realized he's not a dynamic speaker. his debate performances were blah and his foreign policy shortcomings began to show. his campaign staff (primarily Scott Wiley, the guy Trump hired) pissed away all his money on overbuilding a campaign staff and he ran out of money. He left the race unable to raise any money.