I suspect it’s not just about what people think, it’s also about how strongly they feel about it, for it to override other factors when voting.

I suspect that a lot of people who on balance support legalisation, don’t feel so strongly about it that they won’t vote for someone that opposes it. Conversely I think a lot of people who oppose legalisation feel about it strongly enough that they just won’t vote for someone who supports it. Politicians are probably well aware of this.