For some background on the design decisions, here's the 'Key design constraints' from https://github.com/sparkslabs/microbit-prototype/tree/master... (the technical specs of the microbit prototype, written in Dec 2014) :

> Programmable - The device must be reprogrammable

> Simple to program - via a graphical browser based UI

> Small - maximum 5cm x 5cm

> Cheap - manufacture cost must be approximately £2 or less when produced in large scale - for example batches of 100,000 units