Its simple.

The amount that (back in the day) used 1024x768 was scarce compared to those using 800x600. There were still a good amount, but they just used 800x600 more.

Now, its the exact opposite. Imagine the numbers, but reversed. There will always be people using 800x600, but a majority of designers deal with professionals, and people who are in the market, and people who want/need a website. Those type of people don't use 800x600 (usually, i understand that is a broad statement, but i support it nevertheless) especially if you deal with businesses.