It's partially because like others said airlines are becoming more efficient at calculating their passenger loads....
But the other side of it is that it's a gimmick to get you to buy your ticket immediately, if they told you there was an empty flight available you wouldn't rush to buy a ticket, but airlines are using marketing ploys to "hype" up their products.
I tried to buy a ticket on a site the other day and there were 88 left at X price but by the time I got through checkout the X price was no longer available and my only "options" were slightly more expensive tickets.
I then waited 24 hours and went back to the same site, 88 seats available at X price again.
This wasn't some no name airline website, this was Orbitz and it infuriated me that they're "legally" doing shady stuff like this. I use "" around legally because I'm pretty sure I could prove it's a bait and switch with what I encountered, now actually getting the authorities to do anything about it is a totally different issue.