Demand mostly. Demand and customer retention.
Walt operated a park that, at it's busiest, saw half as many guests as the Disneyland of today. A lot of what Walt said about operating a park, just doesn't apply. And even in Walt's time people complained about the prices being high, the crowds being too high, and the general quality of the place.
It isn't a straight one-for-one comparison. Climate change, for instance, can drive up the costs of some goods (like paper products) while also reducing the demand of international visits (thru air travel). Cutting a free service like the Magical Express, would make financial sense if those funds were being diverted to other projects, like building a solar farm, that would have a greater positive impact on the resort as a whole.
It isn't really a matter of whether they buses are still running or not, because they are. It's just that Disney isn't paying for it any longer. At it's core, it's just a price increase.
A lot of these companies went under because they insisted on continuing to do business as they always had, and steadfastly holding to their old business models. Companies need to adapt and change to survive, and sometimes, that means abandoning the past.