I have no inside info but this is why I think WDW has switched to the low floor buses. It's what's available. It's the standard for commuter buses. I don't think it has anything to do with steps or no steps,
ECVs, strollers, sitting, standing. It's the basic design that the market place has for sale for mass transit use. The low floor design does load easier, handles ECVs better, etc. Just about every city that uses buses for mass transit has switched to the low floor bus because it makes sense in many ways. Yes it does have fewer seats but the main function of short distance mass transit is to move people from point A to point B. The passenger comfort factor is secondary.
The older bus design was just a variation of the standard bus being used 30/40 years ago. Those buses need storage underneath, commuter buses don't. Tour buses have a much better ride because they're traveling long distances. Commuter buses do a lot of stop and go and have a stiffer suspension therefore the ride is not as good but the average ride time is about 15 minutes. And after all that they're more efficent for various reasons. The reality is that all of the high floor buses will be gone and all WDW buses will be the low floor type. I've heard differing time frames, but my guess is that by the end of 2011, the older buses will be history.
I guess Disney is saying, this is the type of bus we've determined is needed to do the job we need it to do. If it's not something you can live with, sorry, you'll have to make other arrangements. Just my opinion.