Because it's very dangerous and first responders access in national parks isn't always easy. You can obtain a permit to do it, however, see this memo that summarizes the current situation:
https://www.nps.gov/subjects/policy/upload/NPS_Guidance_Memo...
The legal history is a bit more complex. TLDR: assholes that were BASE jumping in Yosemite in the early 1980s did things like throw burning barrels off the top of El Capitan and take trucks on trails not designed for vehicle traffic.
Comparing the number of BASE jumpers (small thousands) and the number of hikers and climbers (millions) BASE jumpers just can't have the political influence for access.
Random note, Brendan Weinstein who posted here on occasion recently died BASE jumping: https://www.reddit.com/r/SkyDiving/comments/1q6n7v2/brendan_...
Maybe just refuse to try to help them? Why can't we let people win darwin awards anymore instead of criminalizing it? The people doing this are adrenaline junkies who often would LOVE to die this way if they had to. That's why they are doing it.
For similar reasons, suicide should not be criminalized. Yes I am serious.
It seems danger and first responder access would also be a problem for free solo climbing, yet that gets a pass.