I remember those Milwaukee carts being even larger than the big Rubbermaid cart. If anything, that would have been the downfall of those. The bigger Rubbermaid cart can be a PITA on some jobsites, especially in hallways. I wouldn't want to push around anything bigger.
Cost wise, they were likely in line with Rubbermaid carts.
These things are mainly purchased new by businesses, not individuals, hence the pricing structure. Pricing and retail distribution would be the other reason for the Milwaukee cart failure. I remember looking at them.
The new $400 metal Milwaukee cart might do better with distribution in supply houses than in a retail store.