For the same reason you don't show all your cards in poker.

Sometimes the do (or did) release a single before hand to stir up some buzz, but you don't want the consumer to have a good idea what they're investing in.

Some people are going to buy the album the day it's released no matter what, just because of brand loyalty. If you release two singles before hand and the band has changed their sound or that album just sucks, you're going to lose sales from your main base.

Back in the day, albums and singles were aimed at two different audiences. People who bought albums almost never bought singles (except for b-sides) and vice versa. Singles people are always going to buy singles, seldom albums. If you tip your cards too early, you risk cutting into your album sales.

Same with the middle ground, you want them to buy the album because you make more. The promise is there's a whole album that sounds the same and if you release too many singles before hand, they know that's not the case (I'm looking at you Pablo Honey).

If a second single off Pablo Honey had been released after Creep and the music buying public realized that song was the exception and not the style of the album, the album sales wouldn't have been as strong as they were releasing the album first.