Here's why:

1) Innocent Americans -- e.g. Otto Warmbier -- might well be murdered by the N.K. regime.

2) NK can take Americans hostage and use them for strategic leverage.  There are many examples of this happening in the past.

3) NK uses the revenue from tourism to develop their WMD programs which pose an existential threat to the West.

Of course, if you would like to visit NK on an extended holiday, then an exception might be arranged.