dankstersauce gave the best reason.
When I first started using dope and other drugs IV, I had the alcohol wipes, fresh needles, distilled water, sterile cotton filters, hydrogen peroxide and paper towels.
Your attitude towards hygeine changes a whole lot. I still really don't care about it. Water on the skin feels like hell whether you're high, sick or on maintanence.
When you get a shot a the doctors office or hospital, its one time. Even if they brought a Heroin addict with horrible technique off the street to take a ml of blood with a dull needle, it wouldn't be a big deal: it would be a tiny red dot that would heal. If you went to the doctors office or hospital 3-10 times a day for a shot, despite perfect hygiene, you would get tracks after weeks, months, years of doing this everyday.
Multiple (and I mean multiple) needle pricks a day, especially if you miss sevral times or can't hit a vein after multiple tries, combined with dull needles, lots of bacteria infected cottons, water/liquid, etc, and careless or poor technique (another thing that changes with time. Vein rotation, technique, etc doesn't mean shit when you're sick and want to get that shot before you piss/shit/vomit all over your self).
One of my bad habits was once I put the needle in, I'd dig around until I got in the vein- usually this meant I went on too much of an angle, leading to a needle bent on the verge of breaking under the skin trying to maneuver it blindly, shaking and anxious/frustrated into the ever elusive vein. Shit like that leaves black and blue bruises, bright red spots, pus collecting, etc.
IV Cocaine is the worst offender. Needing a new injection every 30 seconds-10 minutes until you're out of coke, plus cokes effect as a local anaesthetic, plus the nature of large doses of stimulants on the CNS leading to severe shaking, unnatural jerks, etc. IV Cocaine does a hell of a lot more damage than other commonly injected drugs.