I'll chime in here.
I too, when I first came upon Sublime Text, asked "why so expensive?"
For me it was a big deal, I am a pensioner and only code part-time to keep my mind active but after only a few days I changed my mind.
I am using the beta of version 3 and there is NO WAY I could ever go back to something else.
If you only want to edit text then fine go somewhere else but if you want a tool that does ALL that ST does, and as fast, then there is no option.
This is without a question one of the finest pieces of software I have ever used and I will, and do, tell any who listen to use it.
So to answer the question posed in the OP:
Because it is simply the best at doing what it does and if you think it is too expensive then you are not using it for it's right purpose.
Put it this way. Would you buy a Ferrari if you were only going to drive to the shops and back? No. You'd get by with a Mazda or something.
But if your neighbour said "here use my Ferrari, and if you want you can help with the fuel costs, otherwise no problem." Would you drive it and complain about how much fuel it uses? Or say "Wow. I'm driving a Ferrari."
Sublime Text == Ferrari
good neighbour == developers
complaining user == ?