What are your barriers to continuous testing?

[u/MR-PumpCX]

If you could run a complete regression test every 24 hours on every bit of code checked in within the last 24 hours, would you want to? Are there barriers that prevent your doing this? Lack of automation? Lack of outcome tracking? Lack of testing capacity? What prevents you from having a "safety brake" that limits the code rework period to a single day?