Why does it take a unilateral posterior crossbite up to 2 years to correct with invisalign?

Arch development is the answer and the safest and most comfortable method is epigenetic orthodontics utilizing DNA Appliance

Correcting any posterior crossbite using Invisalign is much like trying to hammer a nail with a screwdriver. It is simply NOT the best nor most efficient way to treat the problem.A much better way to resolve your posterior crossbite is with an arch development appliance. The crossbite can be eliminated in less than a year and then your teeth can be straightened with Invisalign.Good luck!

Generally, invisalign is a good appliance to correct posterior cross bite in adults since we could expand upper posterior teeth and at the same time constrict lower posterior teeth. It should not take 2 years. However, especially in adults some severe posterior constriction of the upper arch will require surgical correction and invisalign will not be effective. So it really depends on the amount of constriction  and age of the patient.