According to Daniel Nenni in “A Brief History of ARM Holdings”
Acorn sold computers to schools and so cost was a prime concern, this meant that when it came to replace the aging 8bit 6502 in the BBC Micro with a more powerful microprocessor it had to be cheap.
Unfortunately the commercially available alternatives at the time were simply not cheap enough and so Hermann Hauser, the Managing Director of Acorn, decided that Acorn should build its own 32bit microprocessor – however he gave the ARM design team two distinct advantages over other microprocessor design teams – no money and no people! So the design had to be simple and straight forward, indeed the first ARM reference model was written in just 808 lines of Basic.
Interestingly, although the ARM silicon worked first time, it appeared to be consuming no power at all, at least that is what the ammeter said. It turned out that the test board had a fault that meant the ARM was effectively unpowered and was running solely on leakage from the I/Os. This low power consumption was a valuable side effect of making the ARM cheap and turned out to be the key to its success in the emerging mobile electronics market.