Most likely the lack of compost. No substitute for good ole home grown compost!LOL
You and the others that planted that brand of tomatoes have convinced me that I will need to plant some of those next year. This year is the first year that we have planted a bunch of romas.
They have done well too, except for the blossom end rot. None of the other tomatoes had that even though they had the same soil in the buckets.
Any way, I am saving some seeds from the romas to plant next year, and I am going to try to grow some in a pot indoors this winter.
I had the same problem with BLR on the San Marzanos but many escaped the BLR on the same plant(s) and produced lovely big tomatoes. I don’t know why some but not all got the BLR.