It is a good question. A better one might be, why were tactical nukes under consideration?
There has always been a lot of contraversy over the use of submunitions, it continues today. Some is due to confusion - if you are familiar with the construction of some of the smallest submunitions, BLU-43/44 dragons-teeth, the XM series of gravel mines - these are munitions 1-3 inches across, made either of cloth pouches or plastic bits that look not unlike maple seeds. During this period both were in heavy use, and it was also about this time that the US came under accusations of sowing explosive "children's toys". This caused public outcry and heavy scrutiny over many programs, especially as the war and many of the programs (use of riot control agents by the ton, accusations of the use of nerve agent, etc) were already under fire at home.
If you recall, one of the biggest military complaints from the war was that it was decided and fought based on politics. I would suggest that at this stage and with this program the situation was the same. Any decision required approval at the highest levels, lest there be political repercussions. Don't know for certain that this was the case, but it would seem consistent with many of the other things that we do know from this period.