The answer to your question may be simple. Did you think to ask your new orthodontist the reason for the change to thin wires from thick ones? That would be your best source for an answer.
If that is not possible or if you prefer not to ask the orthodontist his reason, here's some answers for you:
(1) The new orthodontist may have seen that the initial rapid movement of your teeth had been accomplished so thinner wires could be used. You had your braces connected with thick wires for at least 18 months. Initial movement and repositioning is more often accomplished at the beginning of treatment. Braces with thick wires may be preferred by many orthodontists for the initial rapid movement of teeth and for guiding teeth into place.
You didn't say if you went for regular adjustments and checkups over the 18 months while you looked for an orthodontist you liked. Evidently you liked something about this new one or you wouldn't have allowed him to perform adjustments or treatment.
(2) Your heavy wires were in service for 18 months and they lost much of their original properties over this time. Adjustments require the wires to react predictably to changes in tension and pressure. A new orthodontist wants to start his treatment knowing how your equipment will respond to his adjustments. Thick, tired old wires lack elasticity and other properties after being in service for 18 months. New wires allow predictable adjustments of tooth movement.
(3) Orthodontists may prefer to work with thinner rather than thicker wire. If all else is the same, your new orthodontist may prefer the properties of thin wire to thick wire. Thin wire is easier to adjust and its more comfortable for the patient to wear. Replacing older thick wire with thin wire for a new patient who has had the thick wire in her mouth for 18 months is a good way to make a good initial impression on the new patient. After 18 months the thick wire needs to be replaced anyway. It’s done its job of heavy moving and thin wire is preferred by patients and doctors alike.
If you had returned to your old orthodontist after 18 months in heavy wires, he too may have switched to fine wires in order to allow him to perform the finer tooth movements needed to finish your case. Most likely it was time to switch to thinner wires for more precise movement of your teeth into their final position to finsih your case.
These are a few reasons for thinner wires. Again, your best bet for a good answer is to ask the doctor who replaced the thick wires with thinner ones his reason for doing so. For all we know he may only work with thin wires.