Why are heirloom/organic/conventional asparagus seeds so rare?

Asparagus benefits more than most vegetables from hybrids. Hybrids have been selected with much lower "female" ratios, making for more productivity and less weeding. That coupled with the long maturity period and success rates of growing from rhizomes or crowns cause seed asparagus, especially heirloom, to be relatively rare.

I noticed this, too. I have, in the past, occasionally seen Asparagus seed in the stores. But this year, I couldn't find any anywhere! I went ahead and bought crowns and they are doing well. But, I was hoping to do this bed with less money.