Like I said, it's just a guess. I believe Morrissey when he says he doesn't like Ian's voice. I also think-- and he has never talked about this to my knowledge-- he would regard Ian's lyrics as rubbish.
Mostly it's about style. In the few years before The Smiths formed, Joy Division were seen as dour and colorless, trenchcoated precursors of the "New Romantics", and if the first year or two of The Smiths are anything to go by (summed up by gladioli all over the Hacienda floor), there couldn't have been more of a contrast of aesthetic sensibilities. Joy Division just weren't pop music, nor were they really punk, and Morrissey has never liked that sort of thing: long songs, unusual structures, heavy production, synths and drum machines, a more avant garde flavor of despair and alienation, and so forth.
. I can remember a few more details but it doesn't seem to be helping me find the interview, unless it's been removed or whatever. Bernard started the interview with a joke about carrying Ian's ashes around in a box 
