Because there aren't enough LVTs, not adopting this change places every practicing veterinarian in the position of having to choose between either flouting the rules or offering diminished care.  Additionally, this was the accepted standard of care for decades with no significant detriment to the patients in Virginia.  All assistants are under the appropriate supervision of a veterinarian who retains full responsibility for the care and treatment of patients under their charge.  Unless and until there are sufficient LVTs to meet the demand, this change is simply one of obvious practicality.