Why is it called "1999 Mode"?

I unlocked 1999 Mode for Bioshock Infinite, and I'm a little curious why they chose to call it "1999 Mode" instead of "Very Hard" or something like that. What was so special about 1999?

It's intended to be a difficulty that hearkens back to the "hard old days" of FPS'es, as Ken Levine explains:

We call it the 1999 mode because our first game came out in 1999, System Shock, and it was a tough, tough game. We wanted old school fans to have a sense of the game that was pretty unforgiving, as BioShock 1 was not particularly unforgiving. I designed it to get through it. We really wanted to give gamers that kind of challenge who wanted it, but it’s tough stuff. It’s like the hot sauce on the table, use it with care.

The sense that I get from reading multiple interviews with Levine is that he views the "modern" shooter as having been made easy for the masses. They wanted to give the hardcore player a chance to play a more challenging, strategic game.

"I really had to get back into the brain I had in the 90's," Levine explained. "It's that old-school feeling of 'If I fail, I deserved to fail' instead of 'the game made me fail'".

They hid the 1999 Mode option (either by entering the Konami code or playing the game through on some other difficulty) so that the average gamer wouldn't stumble on it by accident.

It's a reference to System Shock 2, which was released in August 1999.