the sportwagon is famous for been the least practicle estate ever made.
even parkers say it is "Not as practical as 'traditional' estates", and "This is a 'lifestyle estate', so it means that it's trendy, respectable to have on the drive, but lacks the interior space of a traditional load-lugger."
it is more of a hatchback than an estate. if the GP SW made the same mistake it would suffer the same problem as the sportwagon, who would want to buy one unless they preferred the styling? each model needs to be aimed at a market segment, who would prefer an unpracticle GP SW when a 5 door hatch GP is just as good and looks better?
the saloon 156 also has the least usable boot i've ever come across on a saloon, even getting smallish items in the boot is a real struggle. the boot is an ok size but the opening is too narrow. that gave the sportwagon an advantage because a hatchback was not available, if you wanted a 156 and needed a usable boot then you have to get teh sportwagon. a GP hatch is available so a GP SW would not have the same advantage, it would need to be a true estate to differentiate from the GP hatchback.