It's not steep in the value it provides, or the amount I've used it. But it's steep enough that it's more than I often pay for one thing. It's steep enough to make me come up with some excuse -
"well, it's optional..."
"I can't afford it this month, maybe later"
"It's good, but it's not $70 better than other text editors"
"I'm mostly using it for work, so I'm not going to pay for it out of my own pocket" (and the company isn't going to pay for the above reasons)
"It's not open source, and I don't want to contribute to proprietary software"
etc...
I think if it was a limited free trial, the price was cheaper, or if they had some sort of subscription model, they'd probably get more money from people. Personally, I put off paying for it when I was poor and unemployed in the past, and now Atom is a decent open source alternative, so I tend to use that.
Until Atom existed, if you'd given me Sublime for a month, then told me to pay up or go back to using Scite/Gedit/Notepad++, or learning Vim, I'd have dropped the money on it in a heartbeat