A particularly bad (but probably not uncommon) reason not to support MMP
"If this thing passes then we'll be the only province doing this you know."
Yes, that was the argument against MMP. It may seem silly to a political junkie, but that may be a fairly common sentiment to those unfamiliar with the topic. Talk to everyone you know if you support MMP, people may hold to reasoning like this. Once you take the time to explain the benefits of MMP though, they often come around.
MMP is, I think at least, fairly simple, but it is new and perhaps unfamiliar to many in Ontario. If people have the "it ain't broke, don't fix it" attitude (perhaps not realizing how FPTP is broken since it gives majorities to parties that have lost the popular vote), then they may not be interested in changing the current system for something that is new.
MMP is simple and effective, let's let people know.