The vocalist is the most visible band member, almost without exception. They're frequently the frontman, and a lot of more mainstream bands tend to emphasize vocals over other things anyways. And it's easier for our ear to tune in to the vocals when everything is getting equal emphasis anyways.
I would say there's a psychology aspect to it, too, that has to do with speech. If you hear someone's voice, suddenly there's a jump and you get a sense of personality and humanity from them. Imagine the party or the group meeting where the one guy you've never seen before is there and doesn't talk for like half an hour, then when he starts talking you finally notice him, even when he's finished. Something about speech makes us look at someone and go "yeah, them".
Another point might be that since the majority of the population doesn't play an instrument but has tried to sing a lot, we can generally more easily recognize outstanding vocal talent than outstanding instrumental talent, because most of us (myself included) don't know what all really goes into playing an instrument well. And also since most of us don't play instruments and so aren't as finely attuned to, say, a guitar's range of timbres as a guitarist/musician would be, it's noticeably harder to distinguish different styles of guitar playing, even when two different people play the same thing back to back. But we can very easily tell the difference when two people sing the same thing back to back. Our ears are very well-attuned to vocal differences, since we talk almost every day to different people and learn to identify their voices, but our ears aren't as keenly attuned to differentiating instrumental styles since we don't communicate in guitar riffs, though I think we totally should. Anyways, the trouble in differentiating means that guitarists don't stand out as much any more than that one bush does from that other bush (unless you're a botanist), where vocalists stand out a lot more.
Last point. The vocalist is, if anyone in the band, the one who will be free to move around on stage and, well, do stuff. This doesn't apply if they also play an instrument and have their hands full. If they just sing, they have both hands free for performance things and aren't weighted down/constrained by an instrument so they can move very freely, even moreso if they're wearing a headset rather than carrying a traditional mic with them on stage. This lets them have much more personality visible on stage, so we focus on them more at live events, and thus connect more with them than the others musicians for the most part.
Again, there are always exceptions, but I think between various mixes of the above you probably have your answer.
