You'd have to ask somebody who stigmatised the use of needles I suppose. For obvious reasons, I'm rather partial to 'em myself :D
I suspect much of it is that so many are just plain squeamish (or indeed phobic) about needles in general. Certainly doesn't help anyway. Although - as it happens - I was incredibly needle-phobic... right up until I started sticking 'em in myself on a regular basis. Had to get somebody else to do it for me at first and couldn't even look... then addict mentality took over cos obviously you're expected to donate a bit to the fella doing the injecting so didn't take long to work out there'd be more for me if I learnt to do it myself. Actually funded much of my habit(s) through 'doctoring' for dealers and stuff in the end.
I'd imagine the other big reason is that it's so closely associated with heroin addiction which is such a massive taboo even compared to other drugs and addictions. The phrase is 'dirty junky' not 'dirty cokefiend' or 'dirty alky'.
In truth it's not necessarily more dangerous than other ROAs
per se (assuming you know and adhere to the safety aspects - sterile, fresh pins every time and so on). The main problems are that most drugs are cut to buggery with fuck knows what and you don't really want a lot of that crap going directly into your bloodstream. It gets there however you take the stuff but it gets a bit filtered through things with other ROAs. Other than that there's obviously the onset - if you OD you drop like a sack o' shit with no warning. Happens so fast you don't even know it's happened unless you come around. The real problem is needle fixation though. It truly does become an addiction all of its own. Once you've had that experience of going from nothing to uberfukked in seconds nothing is ever quite the same. You end up sticking anything and everything in a needle just to see what happens. Another one of those bad roads go embarking off down.
All in all, I can't say I'd recommend it wholeheartedly to anybody. Needle fixation is no joke - there's not a functioning vein accessible anywhere on my body these days. Actually there is but that's probably more luck than judgement. One of very few lines I said I'd never cross that I actually managed to stay the right side of. Would fukkin kill for one or two usable ones in pleasing places just for a treat now and then though