|These tassels have enough pollen to attract pollinators and wasps.|
|Source of image.|
Corn is primarily wind pollinated. 8' is a long way to throw pollen, consequently each plant must produce prodigious amounts.
A secondary factor is that traditional varieties are not so much "varieties" in the modern sense but land-races or swarms-of-similar-looking-individuals. The inherent variability provided adaptability to large year-to-year changes in the weather. This was survival corn, not win-a-blue-ribbon corn at the county fair corn. The genetic variability also meant that tassels were emerging and shedding at different times, further diluting the pollen density.
Modern hybrids, on the other hand, are planted much closer together. A farmer targeting 300 bushels per acre will likely plant more than 35,000 seeds per acre. That is fifty times what the traditional Pueblo farmer will have in his field. (Note: The traditional farmer plants more than one seed per hill but will thin to 1-to-3 plants after they emerge.)
Large tassels are not necessary because there are far more tassels, closer than the traditional dryland field. And the modern corn tassels and sheds all at the same time.
Furthermore, the sunlight intercepted by the tassel is sunlight that is not available for photosynthesis. I would not be surprised if corn breeders figure out how to "eject" the tassel after it has done its job so it cannot shade corn leaves.