It's very likely an attempt to capitalize on the fact that Kharkiv had a large number of disused T-64 hulls that could be serviced and put to good use, possibly with Kharkiv's own T-55AGM package on top of that. The less work that Kharkiv has to do to make a tank, the better the result, as the manufacturing capacity of new AFVs of the KMDB is measured in the single digits per year (or less). It took the Thais something like 10 years to get their 48 T-84s, to the point that they just went ahead and bought VT4 tanks instead from Norinco, who delivered something like 50 in a year or two or so IIRC (on top of all the other orders of various models of tanks they make).
Because Kharkiv cannot make anything new in a very timely fashion, they rely on being able to take old turrets and put them on old hulls, call this an upgrade, and then no one buys it. That said, refurbishment is much easier than new construction, just ask the U.S. Army and KMW who are basically living off it. Also because the T-72 is a UVZ tank, not a KMDB tank, Kharkiv obviously cannot do anything with the T-72. You would need to ask Uralvagonzavod instead, and they seem more interested in begging people to pay for the Armata or at least buy T-90s. Because UVZ is still mostly functional as a tank manufactory, they have actually new vehicles to offer like the T-90M or T-14, instead of simple refurbishments of older vehicles (although they can do that too).
People who would want upgraded T-72s would probably look to foot the bill for a -B3 or something if they don't just buy Norinco tanks. OTOH KMDB is just suffering from compounded decades of neglect/lack of investment and weird things like this are an attempt to capitalize on what they have because they can't make anything anymore. I think the Thailand debacle send up a giant red flag to anyone looking to buy T-84s, and the Iraki BTR-4s had huge hull weld cracks, so both of these makes new revenue streams hard to find. Supposedly there is (or was) some big spat between Ukroboronprom and KMDB about this, with the former saying it's awful and still is a problem and the latter saying they fixed it, but who knows. Something something weld 10-15mm of armor plate over the crack it'll hold etc etc.
Even if KMDB has fixed up the quality of its products, it still has the problem of serial production to get over, which requires money for new staff and new machines, which is not easily found if people do not want to buy your products, for whatever reason. OTOH badge engineering a T-55AGM turret onto a T-64B hull, zero houring the hull and engine at the factory, and calling it good is a much safer option in general, as there are plenty of T-64 hulls and T-55 turrets to be found around KMDB perhaps even on the physical premises. It would likely be quite a decent vehicle, provided anyone was willing to buy it, too.
Unfortunately, no one seemed to want to buy T-55AGM, so I doubt people would pounce on the T-64-55, despite its high potential, probably because it isn't new. As capable as it is, a lot of countries that are poorer want the newest things, rather than making do with slightly improved but still effective old things.
tl;dr KMDB can't work on T-72 because it doesn't have the machines, plans, tooling, etc. that UVZ has. KMDB has tooling, plans, machinery, etc. for T-64 and T-55, so it makes this. This is unfortunate because, to be quite honest, a T-64-55AGM-type vehicle would probably have a very high combat potential if given applique armor similar to T-62M.