One of the problems is that some years ago the EU designated certain breeds of plants as commercial, and banned the sale (outside the country of origin) of those off the list. Typical beaurocratic ignorance, and it led to the wholesale destruction of rare breeds of apples and pears among many others. Another factor is that supermarkets want predictable products, chosen for looks and price rather than considerations of flavour and variety. And another problem is cross- contamination from GM varieties, which are increasingly being promoted by governments (especially the EU) under pressure from agribusiness. Saving and swapping seeds of old varieties, and growing them yourself, is an enjoyable way of waving the number of fingers of your choice* at the control freaks.

*Round our way, it's usually six.