It's not just a question of what expertise the legislative has or doesn't have. Just like software you'd ideally want to work on laws in an iterative fashion. It's incredibly unlikely that a law, especially complex laws like the Affordable Care Act, work perfectly so it makes sense to go over it after you've seen it's effects and make corrections.

In the current political environment that's not really an option, so you pass on power to the executive and states to make changes instead.