The keyboards layouts aren't optimized for learnability, but for speed of typing by expert users. Ease of learning also comes into consideration, but it's far from being the most important factor. Locations of letters are determined based on their frequency in the language, their relationships to other letters etc. So for example when key A usually follows key B, it's best to assign them to different fingers and possibly different hands. And since there are 26 letters in the English alphabet, there is a huge number of pairs to take into account, so it's hard to say why a specific letter is located on a specific key.
Same for QWERTY.
This may be an interesting question to post on English Language & Usage.