It's a bit more complicated than that. Actually, some of the first American convert Buddhists were Tendai Buddhists. For instance:http://en.wikipedia.org/wiki/Ernest_Fenollosahttp://en.wikipedia.org/wiki/William_Sturgis_Bigelow
It's true that the postwar wave of convert Buddhism was led by an interest in Zen, and Tendai (among others) was largely unknown. This likely has to do with historical factors that have little to do with the doctrinal issues Chappell emphasizes, and more to do with which teachers and texts are available in English at the time. Zen Buddhism flourished in North America in no small part because Zen materials and institutions were present in North America.
We should be careful, too, because Chappell is talking about the TienTai teachings of Chih-i, not the teachings and practices of the Tendai school per se. (Chih-i was never a leader of the Tendai school, which the OP seems to imply.)
That said, I agree with Chappell's point and the OP's, that the TienTai teachings are indeed compelling and very relevant to contemporary life in North America. I find them compelling and interesting, and I know others who do too. Now that we have Tendai institutions developing and growing here, there is an opportunity for this tradition to flourish here. I hope it does, along with many other compelling and relevant traditions. Let a hundred flowers bloom...