[DISCLAIMER: I'm just going to preface this by saying that I worked in Monorails for a year and a half... have been a member of the Monorail Society for 3 years, and am an avid Monorail fan, as well as a general overal transportation nut. I just finished a rather LARGE debate over on another board on this very subject, and am kinda exhausted about it... so please forgive my brevity.]
While indeed money plays a major factor in the decision not to expand the monorail, it goes much further.
A monorail expansion could be quite expensive, ranging from $500m to $1 billion, depending on size. (And yes, I do have numbers to back that up) That's a heck of a lot of money, especially for a non-revenue generating item. (By comparison, DAK cost $800m to build)
But there are many other factors. When considering such a system, Disney would want to invest thier money wisely. As such, it would install the system into areas where there is high traffic. This would include the All-Stars, Port Orleans, and Caribbean Beach. Park to Park transportation simply does not have the demand to warrant a monorail expansion. Now, since the resorts themselves are the ones that pay for transportation, how would this work? Obviously a brand new monorail system will be more expensive than a bus, and someone's gotta pay for it. This would mean that the All-Stars would face a rate hike... And while we're on the topic, why should someone spend $300 a night for the Contemporary, when they can spend $75 a night for the All-Stars.
The second point... take for example CBR. There are 7 stops within CBR... A monorail wouldn't be able to do all seven stops... it would be able to do one... How would folks get to and from that central monorail station? CBR is too large to hoof it... Same with Port Orleans.
Now, where would the monorail go? Most likely, it'd go to a central transfer station, where guests would transfer to where they want to go. Not a good idea. Keep in mind, we're dealing with people from all around the world, and English isn't always a well known language. And, unfortunetly... even for those of us that English is our first language... the concept of a "transfer" is mind boggling to many. As it stands right now, transferring to the Epcot Monorail is something that a lot of people have a hard time grasping. Guests do not want to transfer, and they get irate when there is no direct route. Not to mention, during peak times, guests would now have to wait in two mammoth lines.
Next... We already encounter this problem on the MK Resort Monorail... if done on a larger scale, it would become more of a problem. The problem with a fixed guideway system and WDW, is that our traffic flow is one way. In the morning, EVERYONE is going to the parks. If you take a look at the MK Resort Monorail... the Contemporary folks fill the seats... the Poly folks fill the standing room... so by the time the train gets to the Grand Floridian... it's full, and few people can get on. So, now folks are forced to wait for the next train. Same problem with that train. And so forth.
With a flexible system like a bus... if a bus fills up, the next bus can bypass the previous stops, and start off right where the other bus left off. And that other bus, can just head right to the park. Not so with a fixed guideway system that must follow its route.
Now, I know most people do not particularly care for the bus. However... it's direct... and it's a lot quicker than any fixed guideway system would be with the required transfer. And THAT is exactly what 90% of WDW guests want. Direct and Expedience.
We think they are a great perk to get from point A to point B. Besides, when youre at Disney, even the buses seem magical 
