How has the world ran out of IPv4 addresses if the 240.0.0.0/4 netblock is still unallocated?

I hope I'm not asking this in the wrong subreddit, but I was wondering about IPv4 exhaustion and the unallocated 240.0.0.0/4 netblock. All of the regional internet registries have ran out of IPv4 addresses to allocate since a few years ago, but I noticed that there is still a sizeable block of IP address space that has yet to be allocated ( / 240.0.0.0-255.255.255.254 ) which is around 6% of the IPv4 address space or around 251.5 million IP addresses. Why does IANA not just allocate IP ranges from this netblock to the regional internet registries so that there would be more IP addresses to allocate?