I think the GOP as forgotten how to legislate
The process involves careful planning and compromise, something no longer in the GOP toolbox
In order to pass tax reform you need to appease both those wanting taxes slashed to the bone and those who want it to be deficit neutral
The GOP Obamacare reform bills were heavily compromised. But, the GOP couldn't pass them because Democrats wouldn't compromise one bit. The party who wants the status quo is under no pressure to compromise.
Another reason the GOP has trouble passing bills is that GOP bills tend to be anti-middle class. The GOP health bill did nothing to make medical care more affordable to the middle-class. This GOP tax reform bill is basically just a tax cut for the rich, which means the middle-class will have to pick up the slack.