Two things in my opinion, the market for Morris Minors is around 20 times larger than the Marina, with around 20,000 cars surviving, compared to around 600ish Marinas & Itals. So it's economic for a manufacturer to make say wings knowing that there is a steady market.
The Minor had gained classic status in the 1970s and so is well established. By comparison, the Marina has only claimed classic status in the last say 5 years and no business has recognised a money making potential sufficient to warrant the large investment cost of tooling up for parts solely for Marinas.
Your not going to spend £10,000 or £20,000 if you can't see the market to get your money back with a profit.
The club have commissioned some panels, but as the demand is relatively small the cost per unit is higher.
Chris
http://www.marinacoupe.co.uk