Because back in the film days, uness you were shooting slide film, your exposure coulde be way off and you'd still have a reasonably good print. People didn't realize how much they were off by, the photo lab fixed everything for them.
Colour film you could probaby be off by ~2 stops and be ok, B&W you could probably be off by ~4. Sure you'd lose some detail in the shadows if you underexposed but most people didn't miss it there anyway. You were probably better off if you over exposed a bit.
Now with digital, it's the opposite, blown highlights are much more noticeabe than blocked shadows and you're better off in most cases to under expose by a bit. The result will be some increased noise in the shadows but that's preferable to blowing out the sky or a wedding dress.
If you shoot raw, you have a lot more flexibility. If you shoot jpg then you have to be right on, similary to shooting color reversal or polaroid.
Here is a photo to demonstrate how much you can under expose with digital and still maintain detail in the shadows. The top image is the ooc jpeg, metered for the sky (part of a bracketed set). The bottom image is after adjustments in LR3 and PS. Had I exposed for the shadows, the sky woud have been blown out and I would not have been able to recover many of the highight details.
Remember the good old days of film when you actually had to know a thing or two about getting the correct exposure? You had to know the latitude of your film. Now we have wider dynamic range and better meters, and still somehow there's always problems of overexposure and underexposure. I don't get it.