Because there is a majority of voters (at the time of this post) who want images available in .sigs (an even larger majority if you discount the "1-2 lines of text, no images" votes; those people are really voting for "No sigs at all, ever").
Oh, dear. Silly me, I thought I was voting for 2-line sigs. What WAS I thinking?!? Good thing I had you here to set me straight about what I REALLY meant.
Allowing max 2-line .sigs invalidates most of the Dwarf Fortress-themed ASCII art that people have in their .sigs.
It prevents the "Think outside the Box!" sig, it prevents the "Adventurer in a Fort" sig, it prevents the "Urist is surrounded by Humans and looks confusing" .sig. I think you all know which ones I'm talking about, because they've been around for a while and are among my favorites.
These are all low-key, ASCII-art, no-kidding DF-related stuff (though Aquilllion might [accurately?] point them out as being "off-topic" and therefore wasted bytes).
How can you vote against these things? That's like being against Elephants and Mom's Plump Helmet Biscuits.
Now, I do admit that the overlap between the "Don't want .sigs at all" and "Just want highly-regulated .sigs" is probably not 100%; accordingly, not everyone who voted for "1-2 lines, no images" doesn't want to totally elimate all .sigs, everywhere. So, yes, I apologize for exaggerating [slightly]. However, the overlap is certainly not 0%, either; I think we can accurately predict which option Aquillion picked, for instance, and (s)he's indicated a preference for the removal of all .sigs, everywhere, as wasted bandwidth and screen real estate.
Begging your pardon, but might you please deign to notice that those wanting to eliminate sigs all together would have an opinion (if in fact that's what no-images-1-2-lines does mean)? Just as much as those wanting to add image sigs where there were none before?
Yes, and the option to completely eliminate .sigs from all posts is already present as an option. Since that group's needs* are already met, there's not much more that needs to be done for them.
* Other than those who want to prevent others from having or enjoying .sigs, of course.
EDIT:
Regarding Toady's latest post, and opt-in for .sig images is just as acceptable as an opt-out; I can't see any issues that would arise should the default setting being easier on those who find .sig images completely distasteful and those who aren't logged in.
I also agree that this forum is not 4chan or Something Awful or ... etc.
As for rules on .sig "tastefulness" (text or otherwise), I like EN World's Grandma Rule (though fouler language is allowed here than is there, generally speaking).