Except for dedicated servers for certain enterprises (they started in enterprises so no surprise there), I think that STUN traversal in CGNATted areas may be the reason - Zoom has no control over how does the NAT machine on the other side works, which on some CGNAT equipments each outbound IP address are limited to a certain bandwidth (to evenly share the data throughput of the equipment). If Zoom has a limited range, then CGNAT users may suffer from limited bandwidth even though Zoom's network may handle this just fine. If they have additional IPs, they can give each participant an endpoint and circumvent whatever restriction there are on the ISPs' side.
TLDR: Except for companies requiring a dedicated proportion, probably to work-around CGNAT problems.