Well, Dave would have to tell you the technical reasons why we can't.
From a practical standpoint, the pollbooth isn't meant to accurately/scientifically measure anything - it's just a way to get us conversing about a topic, and it's kind of fun to know if you're in the majority, or "odd man out" on various topics. Have you noticed how sometimes a person you might have thought you have nothing in common with leaves a comment that is exactly what you might have written? Poll questions can draw us closer together, and let us find out more about each other - things that we might not think to tell one another unless somebody asks.
Since non-subscribed members can also participate, it's often their first interaction with DG subscribers, and hopefully they see we're a pretty fun, lively bunch of gardeners here! ;o)
Short answer: It would take much more time to program.
Longer answer: When I first made the voting booth, I didn't think anyone would ever want a multiple-selection capability, so I programmed it to only accept one answer. To change that now would mean almost re-writing the voting booth from scratch, which I really just don't want to do, since it's working so well. So,it's a matter of priorities. :)