The more trim levels and stand alone options you have, the harder it is for dealers to figure out what to order and what to sell. Then, you have a whole bunch of cars with features no one wants sitting on the lots losing money, that dealers now have to deeply discount to move.
Al a carte options packages only seem to work on premium cars, like BMW or Porsche. The customer doesn't mind ordering their custom premium car and waiting 6 weeks for it to be delivered... but once you cross under the $40k threshold, especially with volume makes and models (like Accords, Escapes, Corollas) customers want the car on the lot that day! This is typically because buyers in that demographic have a 1 car household and not having a car for 6 weeks will really cripple them. Plus, don't forget the incentives from dealer sales promotions typically only apply to whats on the lot. Even the volume sellers of premium makes are getting away from al a carte options; the 3-series BMW seems to have fewer and fewer stand alone options with each new generation.
Dodge has done a combination of option packages and stand alone options with the Dart and that experiment has been nothing less than a complete failure. Dealers have too many variations of the same car to try to order, and with so many Dodge dealers, customers will just keep shopping around until they find the Dart they want. VERY VERY few customers order the configuration they want and wait 6 weeks.
In Mitsubishi's case in the US and Canada, when selling such a small amount of cars, it would be way to risky to sell too many variations of the Mirage. Because if a customer didn't like how a Mirage was configured, they more than likely wouldnt order one, nor go to another Mitsubishi dealer to find one, they would probably just go buy a Chevy Spark instead.