Is it because the victors of WW2 were Christians too and wanted to defame Hinduism?
I presume because it was an existing known symbol in the world that the english already put a name to it in their own vocabulary (colonialism and all that), it wasn't meant to be some new symbol and consciously included the swastika use in india
/u/commiespaceinvader wrote this excellent post on how nazis (and their predecessors) used the symbol and what it meant to their movement
Maybe he can shed some light on it