So this decision was propably made to mitigate the alleged spam of these, but in reality... nobody uses them. Seeing mortar being used is a VERY rare occasion. Moreover, a mortar being used properly and effectively is almost impossible to see on vanilla conflict servers. That's because of few factors: Rank, Teamwork and reality of conflict mode gameplay.
So the captain rank requirement for these reduces their use significantly. To get to captain without exploiting game systems you need 5+ hours of gameplay on the same server. Which effectively gatekeeps major part of players. Imagine you would like to jump in the game for 1-2 hours of your entire free time. In that time you basically cannot use 50% of toys the game offers, simply because you dont have time to rank up enough in that time. The fastest way to get to captain is to do logistics for 1-2 hours straight. You can also get to captain in 30 minutes if you do that logi in specific locations where you have the shortest route from supply point to base with arsenals. With that system only a handful of people tend to have rank of captain in most rounds. While this system is not the worst, and limiting rocket helis behind it is also not bad idea, locking mortars behing it surely is.
Rocket helis contrary to mortars require only one person to operate. And the fact that mortars require teamwork, also sigificantly impacts their already reduced use. When players reach rank of captain, most of the time they choose the rocket heli, because its much more fun than sitting on a mortar as you get to see explosions atleast. Also practically the only person to operate a mortar (with explosive shells of course) is the one with rank of captain. Which by itself sounds absurd because why would someone with such high rank operate a mortar. And no, using bots to call mortar strikes is not really an answer, because that's not as fun as working together with other people. Everyone should be able to use a mortars regardless of rank.
Next are the realities of conflict gameplay. Usually people are so scattered across the map its hard to find a use case for mortar (I am talking about ~100players on Everon). And you cannot really use mortar for anything but killing other people. You could use them to destroy vehicles, but thats even harder, considering there are less vehicles than people and getting to hit them is hard aswell. If mortars could actually **destroy** base buildings, **not just visually**, then their use would propably rise.
With all that being said, reducing explosive mortar shell requirement (becase that was what I was talking about basically) to **Sergeant**, or even **corporal** would make them much more usefull and simpy more accesible to players, while not impacting game balance too much aswell.