IIRC, the short answer why it wasn't repurposed as additional unicast addresses was that too much deployed gear has it hardcoded as "reserved, future functionality unknown, do not use." Following an instruction to repurpose 240/4 as unicast addresses, such gear would not receive new firmware or obsolete out of use quickly enough to be worth the effort.
More to the point, the amount of work required to fix all the existing equipment to handle 240/4 would not be a lot less than the work required to get it to handle IPv6, and it would only have pushed the IPv4 exhaustion out a few years. It was entirely reasonable to conclude that it would not have been a good use of anyone's time or money. Look at the bright side: you can use the money you didn't spend on 240/4 upgrades to buy slightly used IPv4 space on the grey market or CGN equipment. R's, John