For the exact same reasons that you want to vote for UKIP. Why do you want to vote for UKIP? because you like their policies best ?
BNP is authoritarian centrist, UKIP is liberal right wing. They have pretty different policies on a lot of things. Although I haven’t read a BNP manifesto for several years, so they might have changed.
The main reason I will vote for UKIP over the Conservatives is May’s inability to deal with islam, she described it as a “great faith” shortly after the Westminster attack. UKIP has a different approach to it than the BNP, although Nutall has been pretty terrible at explaining it compared to Farage. An example of this might be that BNP’s policy is ‘Ban the Burqa’, wheras UKIP’s policy is ‘review when and where it is acceptable to cover your face in public’. There is a subtle, but important difference between them.