[sdh9]

You don't really need to type long form text on it. The primary use of the keyboard on the MCDU is for the flight management computer, and aviation fix names are 3 or 5 letters at most. That is the primary design case for the keyboard. ACARS is secondary, and on a typical flight only a few long-form text messages are sent.

Every aircraft that I've ever flown has an alphabetical keyboard. Typically horizontal space is valuable, yet vertical space is less valuable, so it's easy to make the keyboard long but not wide. However, as others pointed out, it seems to be changing in newer jets.