Now over time the cost of the new engines will dramatically reduce as production of new materials and parts is mastered and local production reduces the costs involved, but as you could imagine if you have an existing fleet of aircraft that burn a little extra fuel... take a little longer to get places and need slightly longer runways for takeoffs it is not the biggest priority to spend 24 million dollars on each one replacing the engine... especially for a tank because even though it only has one you will need a new transmission to use the extra power.
It is also important to remember that fuel consumption is related directly to power, so an extra 300hp might sound good, and improved fuel consumption might even make it sound great but effectively more power means more fuel consumption, and there are no guarantees that the improved engine performance will actually improve the performance of the tank by the value of the dollars it is going to cost to upgrade the tank.
ERA is simple and cheap and radically improves the performance of existing armour without adding a lot of weight or other problems.
With a new engine on the other hand... what sort of production capacity do you have... they might not be able to make enough of them to put them into all the new vehicles they are making as well as replacements for problems etc etc, while the other engines being made for the T-72 are already pretty good anyway...
Over time it makes sense to switch over to the new engine and discontinue production of the older models or reserve them for export only for those wanting a cheaper simpler tank.
There could be issues with fitting it too... keep in mind that the T-90 is a different tank from the T-72 so some things might fit easily and others might not be so easy to adapt and use...
Matching an engine to a vehicle is not just a question of fitting the most powerful engine all the time, there are other considerations to think about too.
It is always a cost benefit thing where you ask yourself if the extra cost is going to produce enough of a change in performance or ease of maintenence or logistical advantage to make it worth it... the Russian military does not get money to burn.