This question already has an answer here:
That's basically it, in a nutshell.
There are currently two proposals on the front page of Area 51 with 0 followers. You read that correctly - zero, none, nil, nobody at all. This looks absolutely horrible for starters. Imagine the obvious reaction of new committers visiting for the first time,
You keep a proposal that's so bad even the person who proposed it has no interest in it! Well, I won't bother joining some others!
I can only imagine users do this to avoid the (perceived or actual) penalty for having a proposal closed, be it as duplicate or for insufficient activity or just ignoring basic Stack Exchange principles.
Now, if there isn't any penalty beyond other users' negative social pressure, there's really no reason to abandon proposals, since Area 51 is the equivalent of an industrial manufacturing area: no community stuff really goes on and population is highly transient. If there are other penalties, abandonment becomes the equivalent of spraypainting the license plates black so the car isn't ticketed for reckless driving.
Please, for at least one good reason, disallow proposers from abandoning their proposals.