122mm is an old caliber. European countries went to using 105mm to replace it back in 1930s or so. The reason being that this allowed for slightly lighter and thus more mobile guns with only a minor reduction in firepower. Soviets also pondered on making that change but due to worries about firepower and lack of experience with this ammo, chose not to. Then WW2 came and they made so many artillery pieces that replacing all of them with a new 105mm after the war, was simply not worth the cost. ...And then decades and decades of more 122mm howitzers and ammo produced and sold and this caliber remains as a zombie much like the 7.62x54R. It could be replaced with something newer and better, but they never get around to doing so.

Then you have to consider this: If you want light and mobile (air mobile even) artillery, the 105mm beats the 122mm in that, and at least USA has managed to make 155mm towed howitzers that are just as mobile as the 122mm Soviet design, so the lightness is no longer a selling point for 122mm. AND if you on the other hand put it on a self-propelled chassis, if the weight is no issue, you could go for some bigger guns like the 155mm instead of using a weaker (and shorter range) 122mm.

So the one remaining niche that D-30 has is countries with loads of ammo and production for it may as well keep using it, and if you can give it a light mobile chassis you can use it as a poor man's SPG. 105mm would likely do an even better job now that I think about it, but at least the D-30 is better than the other Soviet towed artillery pieces. The 130mm (I forget the model designation) for example is back breaking heavy and difficult to use for the crew. D-30 meanwhile on towed chassis is surprisingly light and rotates 360 degrees around which in rare cases is useful. But in modern warfare, you'd still prefer it on a mobile chassis for "shoot and scoot."