When they are good, specially at cs1.6, they get made fun of because they have or are "newb" accounts basically, and like the low digits are tEh 1337 oLdSk00l
The reason low digit steam accounts are so precious or wanted by people for a bigger price is because the lower the digit the longer the steam account has been out. You will probably also get into leagues much easier with 4/5/6/7 digits than you will with an 8 digit.
Ex: Account ID: STEAM_0:1:2685 would be worth much more than STEAM_0:0:1785665.