Because TD is a crooked outfit. Just go with a different brokerage and buy all you can. I transferred everything out of TD and moved it to etrade. Need you know more?????
Fly with Etrade, Dude. (But even Etrade will sometimes arbitrarily NOT ALLOW purchase of whatever stock --- even though the stock in question is NOT an official "Caveat Emptor". I also hear that "Interactive Brokers" will let you buy even "Caveat Emptor" stocks --- at least that's what they told me in an email a few months ago.)