Currently, RHV doesn't have an option to use VDDK method to convert the VMware VMs. There is a huge performance improvement while using VDDK to migrate the VMware VMs. In the customer test, the migration time was reduced to 9 minutes from 70 minutes after using VDDK. Also, refer https://bugzilla.redhat.com/show_bug.cgi?id=1927111#c2.
Yes, the HTTP access method (vpx/esx protocol) is not the fastest, but it is easy to setup. The problem with VDDK is that it is proprietary library. It cannot be distributed by Red Hat and its users would have to take care of downloading the library and properly deploying it on the VDSM hosts. In other words it is not just about adding the feature to VDSM and RHV UI, but it would also require proper documentation plus a considerable involvement from the user to get the environment set up. If the customer intends to migrate so many VMs that the conversion time makes a significant difference they should consider using the IMS method.