This is mostly just speculation, and SE has never outright made a comment about it, but it's probably to avoid the appearance of discrimination and coercion within their subscription base.

Not everyone who subscribes to FFxiv has the Heavensward expansion. A lot of the queue times are obnoxiously long, or even dead, but there are still players who run the level 50 content. If SE were to bump any of that content up to 60, they'd be alienating the players who don't have access to Heavensward from content that they was originally intended for level 50. It doesn't matter if that content is effectively dead. So long as it exists, SE can make the claim that they are not neglecting a portion of their paying customers. They can also avoid any accusations that they are attempting to force people into purchasing the expansion by limiting their playable content. The option is there for people to queue for. The fact that the majority of the subscribers choose not to is not SE's responsibility or problem.