[jnsaff2]

Because when they are AC-coupled (synchronized) they need to be either in balance or have energy flows no more than the interconnects can handle. Otherwise the lines will trip out in order to protect them and then both sides will be even more out of balance.

With DC you have in essence giant transistors working at each end[0] and you can control how much energy you want to flow and in which direction.

So the topology of the AC-synchronized network needs to be such that there are no bottlenecks or SPOF-ish things in the middle. With HVDC links you can build those one by one and in whichever capacity you want.

In the case of Ukraine the short answer is: there is enough transmission capacity for current operation. If there is a major event on either side and the capacity of those lines is exceed the circuit breakers will activate and separate (desynchronize) the networks. The active management of power generation capacities tries to keep the energy flows in check.

[0] by end I mean AC/DC and DC/AC conversion point, you could have more than 2 if you wanted to.