So that there's less of a possibility of unlocking the phone unintentionally--simply by taking it out of your pocket while (accidentally) touching part of the screen, or something similar.
We've already seen quite a few posts of people suddenly running into issues with the phone unlocking unintentionally since they've installed iOS 7, with the likely cause being that the unlock action now can be activated anywhere on the screen (rather than in a particular part of the screen like before) and only requires a swipe of about 1/3 of the screen for it to take (rather than at least 2/3 if not more before). Simplifying unlocking even further (like making it possible to do it in any direction) would likely only make that issue worse.
The whole lock part there is to prevent anything unintentional, so unlocking should be something quite intentional (more or less as it was before iOS 7 even).