Busses used to be a great solution to a minor problem...how to get people from a few "non-orail" resorts to Epcot and MK. As the Studios were added, you could take a boat/monorail to most of the theme parks and from most of the hotels, save CBR. Suddenly the expansion of hotels, water parks, and the addition of AK means you now are greatly tilted the opposite way...the vast majority of guests are going to hotels with none of these services.
Bus was always the default and the solution was likely "Add more busses", because its cheap/easy on capex to get approved, known in quality and has is fairly low risk. The problem with this is that unchecked growth in the resort has caused the bus system to become inadequate for the ongoing growth and "add more busses" isn't a logical solution when there are enough busses, just not enough ability to accurately forecast when and how many are needed at any given time. Its easy to plan for the end of the day, because you know how many people are in the park. The start of the day, the afternoon nap time, park to park, transfers, etc...are all random and unknowable. You cant just throw a bus at them every 20 minutes, either. Traffic is a question as well and there isn't a guarantee that an guests will ever be waiting either...busses can come and go with no guests to one resort and, for some reason a full corral of people sit waiting at another resort.
The advantage a monorail or a Gondola is that they are always operating at peak efficiency, all the time (assuming the system is functioning). The problem with a monorail/gondola is that its a $300 Million cost before you even get off the ground. Adding a bus is a $100,000 and an extra FTE. When you are going to a boss and solving a transportation problem, the point where the monorail expansion/gondola build becomes feasible is when the huge ongoing expense of the busses crosses the level of the high upfront cost. It seems that Disney has now hit the point where running busses is just too cost prohibitive on an ongoing basis.
WDW owns a fleet of about 400 busses...lets assume 350 are running any given day and require 2 FTEs per day per bus...using normal Human Resources math, we would get about $23 million per year...on JUST bus driver pay. That doesn't add in items like mechanics, maintenance, bus costs, gas, infrastructure (roads, bus depot, storage, shops, cleaning, etc). Its not difficult to assume the annual expenditure for bussing, alone, at WDW is in the area of $75 million (if not more).
You can tell that this number has finally hit a pain point for Disney to build out a newer mass transit option. Based on the costs of the bus and rising fuel/labor costs, I would guess that gondola stage 2 (or the AK link/gondorail/light rail/whatever) is going to be coming sooner rather than later and is only being spaced out due to the massive capital expenses currently being laid out for Epcot, Studios and gondola phase one.