Reason being, a lot of modern motorhomes, and vans, especially Motorhomes with more berths, 4x4 systems, automatic transmission and other sundry accessories, look like your regular van, but many of them weigh over 3500kg, some knocking on for 4100kg.
Raising the B limit would allow people who want to drive bigger vans (not just motorhomes) without the need to take the C1 license.
It would also allow people to drive bigger delivery vans also without the C1 license test