With respect to MRC, two thoughts come to mind: (1) do "individual conditions" prevent a longer platform? and (2) does the platform there pre-date Amtrak's stated desire for full length platforms?
I don't know a lot about MRC, but if you look at it on an aerial view, it looks like a pretty small chunk of property wedged between the major highway through town and what looks like a UP parking area/equipment lot. There appears to be space enough to almost double the length of the platform, but only if you but right up against the highway. There's probably some sort of law or regulation or zoning or something preventing that. Plans are afoot for the station to be moved, though, so this whole discussion would become moot.
The short platform at Maricopa may have forced on Amtrak as the best of a range of poor choices given the funding situation in the mid-90s.
About the plans for a new station in MRC, found this
local news article from June "Cost of Amtrak rail siding estimated at $4.2M". Turns out that blocking the traffic on a busy state road in the early morning and mid-evening rather than in the middle of the night provides a strong incentive to the local authorities to get Amtrak a new station location. Which would presumably have a 550' or longer platform to allow for a single stop at the new station and better be built to the right distance from the track!
Interesting sidebar: the city council is requesting $300K from the Gila River Indian Community to help pay for the design and engineering. Digging a little deeper, turns out that by blocking the road, Amtrak is also blocking traffic to the local Indian casino, so the casino has an incentive to help relocate Amtrak.