If there is no reason for caps, why wouldn't one of these companies simply remove them, giving them a competitive advantage, and making them more money? Why would one company reject making more?
Maybe capless actually costs them more due to bad infrastructure, and they don't see consumer demand for it? Forcing them to go capless would in that case result in higher prices.
Maybe they form a cartel and have collectively decided to keep caps. But why, if it doesn't actually cost them more to remove the caps? And if it does, then prices would again rise if forced to go capless.
Indeed two companies is not really competition. So why not focus on that, instead of reducing choice, which may lead to even less competition by making differentiation harder?