Oh, man, there are so many reasons to choose opiates over not just alcohol but other substances as well.
First, I hate having to drink and drink and drink to feel buzzed. It takes forever, and I'm not a big guy either. I want to be buzzed now, right now and without having to step out every twenty minutes to take a piss. Then, three or four drinks and sixteen bathroom-stops later, I am drunk only to wake up the next morning and puke my stomach out into the toilet.
But opiates, on the other hand, work quickly without inducing nausea or hang-over like symptoms. (This may not be true for everyone, but is true for me.) Furthermore, opiates change one's perspective just enough without much potential for the anxiety or paranoid-like effects that other substances have. (Again, this may not be true for everyone.) In fact, I feel comfortable enough to drive after taking a healthy dose of opiates. The same is not true, in my experience, after taking benzos, smoking weed or drinking.
And it's true that there is the social aspect to drinking. I wouldn't say that I am anti-social, but you know, sometimes it is nice to spend time alone, put on a movie or some music and relax. Going to a bar, drinking, saying something stupid to someone you care about, making new bar friends you will likely never see again... that sums up my bar-hopping experience in a nutshell, and I can definitely do without all that.