Why are Watt’s more common than VA’s

Simply because a watt is a measure of work done. The real part of V*A is watts. There are exact equivalents in mechanical systems. \$W=\dfrac{J}{s}=\dfrac{N*m}{s}\$ so \${N*m}\$ here is Force that does work through a distance but it can also be a measure of Torque over a distance and that is static(non moving). One is the potential to do work, one is the work itself.

Using watts for reactive power would be wrong because reactive power is stored power and not capable of doing work.

Using VAr - r for reactive is just short hand for the purely imaginary part, instead of using i or j (for you physicists out there).

I'll offer an intuitive explanation instead. The calculations would seem to depend on specifics – maybe if you sketch a circuit that illustrates your situation it would be easier.

You have a single phase power system from the utility. You want to dump PV energy energy onto the power line but also regulate it to standard line voltage.

How do you regulate voltage? Well, you can either dissipate energy by dumping power into a resistor, or store the energy in a reactive load. Clearly energy storage will be more efficient than dissipating the excess as heat, but the problem is that it takes time to store energy (e.g. to charge a capacitor).

So if there is a sharp transient you may not have enough to store the excess energy quickly enough. On the other hand, you can start dissipating energy in the time it takes to switch a transistor.

If there is a transient and the X/R ratio is low (resistive) then energy can be stored. If it is high (reactive) then energy storage of the transmission line will create a DC offset, so you need to dissipate real power.