The only way to offer ESX is through VMware's VSPP program. Their pricing per VM is higher, making it more exclusive to the higher end market, and they also require a VCP on staff, making the entry costs higher for most hosters. Definitely worth it if that is your market.
I believe cost factor is the main issue why most provider not offering it as a service. ESX is an enterprise grade solution and product cost + support is too high as compared to other Virtualization solution. On WHT you will see more Virtuozzo/ openvz/ xen because of the pricing model.