Why does Twitter have a 140 character limit?

Why does Twitter have a 140 character limit?

I'm just curious to know why, years after the release of Twitter, it still
has a 140 character limit? What's the purpose of this? Certainly even with
billions of tweets a day it wouldn't cost that much to up that limit to
2,000 characters. Twitter has millions of dollars of ad-revenue and their
decision to limit the expression of their users hardly seems like it is
worth the compromise.
What other online services limit the expression of their users? And, what
kind of users would seek out such a debilitating limitation?
I was recently considering making a social dating site, something like a
OkCupid -- would a 140 character profile be desirable? Has there ever been
a successful non-Twitter analog to this? Perhaps an ancient childhood game
where kids had to formulate paragraphs with no more than 1.5 sentences?
Could someone help me understand Twitter?