Supporting legacy IPv4 is expensive and enforces inequality which favors those older providers which were able to get large IPv4 allocations years ago.
Any new ISP starting up (or significantly expanding) today (especially in developing countries) will not be able to get enough IPv4 to provide to customers, at least not at an affordable price. They will use CGNAT - that is sharing a single IPv4 address between hundreds or even thousands of customers. The only providers that have enough legacy IPv4, are those in saturated markets where their customer base is static or declining.
The side effect of this is that the connection is slower due to the extra overhead of CGNAT, and because all the traffic for many users originates from a single address it is likely to get blacklisted from services (eg if one user does something malicious or gets infected with malware). CGNAT also creates other problems for the users.
This makes using the internet over legacy IPv4 a quite painful experience in developing countries. People in developed countries where ISPs typically provide each customer their own IPv4 address tend not to care. This ensure that developing countries are held back.
IPv6 is plentiful, putting everyone on a level playing field. Everyone can have their own block of IPv6 and be a part of the modern internet, not an outsider peering in through a slow and congested CGNAT gateway.