The short version is: There should have been air support. There were supposed to be pilots. There were supposed to be unmarked bombers.

They got identified, and their mission was aborted.

The CIA's own, recently declassified history of the event starts with a nearly-600-page document called "Air Operations."

From p. 270:

The cancellation of the second strike -- for on 16 April all of the targets, including restrikes at the D-2 airfields, were cancelled -- turned a long planned, much discussed, tactical air operation into a 100 percent ground support mission for the invasion troops. By denying the requests from the field for a strike to eliminate (or at least reduce even further) Castro's fighters, it forced the Brigade B-26's into an unplanned and unwanted air combat role.

There's also a widespread belief that the CIA was hoping for a widespread popular uprising - that as the militia moved inland, the oppressed citizens were supposed to flock to their cause and throw off the chains of communism. There were briefings to that effect.

But the CIA's report starts with a quote from Grayston Lynch, one of the CIA folks commanding the invasion:

This invasion of Cuba to us was basically an air invasion ..•• All we were going to do with this Brigade was to go in there, take a beachhead big enough to protect an airfield, and .let the planes do the dirty work -- for one week, two weeks, or whatever time it took •... The Cubans seeing these planes just roaming at will up and down the length of Cuba, and Castro unable to dislodge this beachhead .•• that's when they would jump off that fence ... we never, down at the lower level, envisioned or even asked for or wanted -- this spontaneous uprising that everybody keeps talking about.

The fact that they're so eager to deny plans for a spontaneous uprising kinda makes one wonder. But that's the official word.