For starters, the amount is set by the H&S executive, based on fire and evacuations regulations and other factors. Not Towers themselves. It will be in their insurance and licences that allow them to operate a visitor attraction.
You could have 30 new queue lines if you want. But they would not be allowed to put more people into the park in a given day. Because if they needed to evacuate the park in an emergency, those extra queue lines with all those extra people, become irrelevant, infact hinder an evacuation. It does not matter if you have 30 or 60 thousand people, if the place needed to be evacuated in a hurry, all those people are still held up by the same exit choke points and places where crowds could crush, infact, increasing the amount of people allowed in the park based on ride amount, would be a silly and backwards thing to do as it would put more pressure on these choke points. If they could increase the ability for people to exit the park, they might be allowed an increase.