Possibly either self incompatibility (many brassicas won't accept their own pollen), male sterility (also common in some hybrids), or lack of pollinators. The seed pods are distinctive.
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With a shortage of pollinators or self incompatibility, you would still likely get a few seeds, so male sterility is the highest probability.
Breeders have encouraged male sterility because it helps them create reliable hybrids more easily. The only pollination on a male sterile variety is hand pollination.