I"m guessing probably mostly because of their names. They're also both rather important characters in their respective series.......I dunno, but it is kinda fun to compare them, even though they're pretty different characters.

Because everytime someone hears the name " BELLA" they automatically assume its Twilight's Miss Mary Sue. WRONG! Bellatrix Black-Lestrange has been a Harry Potter character since 2000 and her nickname was Bella long before Bella Swan was " thought of" Same with the name " Clearwater and Black"