All it takes is a fear monger with an agenda like Intuit to whisper "Our experts estimate that you will lose $X billion in uncollected taxes with this new scheme" to send congressmen from both parties running.
Ideally, we would see state governments take the lead here and innovate. If they can show meaningful gains then maybe the Federal government will pay attention.
The opposition comes from business interests whose revenue comes from helping taxpayers navigating a complex code and processing.
But there is a huge industry built around servicing the complex tax code, many of whom wont be needed if the tax code is simplified. Just like any other industry disrupted by changing business environment, they will fight to the death to keep their industry alive and necessary.