The flap created for LASIK (and LASIK-like surgeries, such as SMILE or LASEK) heals, but doesn't have the structural integrity that occurs when the epithelium has to fully regrow like for PRK. So that flap becomes a semi-permanent weakness that can be dislocated down the road and cause problems.

In general PRK is still considered the safest laser surgery option, but trades off the long-term risk of the epitheium flap for a much longer initial post-op recovery time. With PRK you have to be careful that there's no hazing as the epithelium regrows, but once it's regrown it's as good as it ever was. So for folks with a high risk of future eye injuries, PRK tends to be preferred (or required, in some instances like the special forces).