The seed companies make more money from developing/selling F1 varieties, which explains a lot.
Some old varieties have stood the test of time and are still worth growing, plus it's easy to save seeds from some of the crop to sow next year, which seed companies don't want you to do - for obvious reasons.
On the other hand, the companies have developed some very good, disease resistant varieties - like the clubroot resistant brassicas, or rust resistant leeks, for example. However, as most are developed for the farmers, they tend to be varieties that will all crop at the same time, ready for harvesting. This may not be what an allotment grower wants, who wants all of his winter cabbages ready at the same time?
So horses for courses
I grow some old open pollinated varieties, but some F1s are chosen for their disease resistance, better yield etc