Why bands like Dokken and Ratt are not popular in the UK ?

Answer 1

The British are snobs and don't like American bands.

Answer 2

Because they SUCK?  I lived in UK through the whole of the 80's and did not hear about Dokken or Ratt for the first time until I moved back to USA in 1991.  We had WAY better bands to listen to over there than this crap-*** candy-*** mediocre American hair metal.