I am guessing, at this point, it simply boils down to desperation. Without replacement, their tax cut effort is in serious jeopardy. Frankly, I can't believe Ryan's now putting his ass on the line for the third time on this. If this too fails, I cannot see how it bodes well for him.
I'll give Trump credit for keeping an arms-length distance from these bills. He's always said "we" or "our" rather than "my" bill. Gives him an out...but also allows him not to have to admit that he understands none of this.