Here is the reason why. A little reductum ad absurdum to illustrate the point

Let's say, we buff the AR to do 1200 DPS with a 100m range and no kick. Incredibly OP, right? Now, going with your idea, let's make it cost 600k ISK each. Seems a fair price for that power, right? HEre is what happens: the people with money (FA, OH, the NS) have metric f*cktons of ISK from back when PC gave passive ISK. The have BILLIONS of ISK. So they can afford to pay for this montrous death machine and lose it over and over again without worry. However, a new player starting out, or a corp without such ISK reserves, cannot afford to continuously replenish their supply of this weapon. So they don't get to run it as often as the high-baller players. Which means when the high-ballers go against the poor players with their far weaker weaponry, guess whose going to win? When one side has a squad with this 1200 DPS AR going against a squad who cannot afford it, who do you think will win the fight? Consequently, who do you think will receive a higher payout, which allows them to continue using this massively OP rifle?

Of course such a weapon will never exist, but it makes the point clear: Balancing on ISK only separates the haves from the have-nots. Those who are filthy rich can run this weapon for days and not feel the sting of its loss, while the poor can only run it every once in a while. This is why protostomping is so frowned upon; even if my squad pulls out proto to counter a proto stomp of say Nyain San, in the end my squad cannot afford to run proto battle after battle, whereas Nyain San can because they have those massive ISK reserves and we don't.

ISK can be a small balancing factor, but it should never be the primary, or even the secondary or tertiary factor when it comes to achieving balance. Balancing on ISK only let's the rich use it and the poor hate it.