It's just that it's also very widespread. Way back in the Dark Ages of the computer era, from the 1950s until less than a decade ago, programmers trained computers to read dates in a six-digit format. Today, for example, would be 011799, or perhaps 990117. No problem with the month and date, both of which can be adequately described in two digits. The difficulty, obviously, is with the year.
Years have four digits.
When 2000 rolls around, many computers may mistakenly think it is 1900. This could wreak havoc with matters large, such as a bank's calculation of earned interest, and small, such as that nifty little feature in your word-processing program that automatically inserts the date into the letter you're writing. Why programmers used only six digits, by the way, is a matter of some small debate. Either way, we're stuck with the problem now. We were hoping you wouldn't notice.