Thank you for your info, Jikan. So the Tendai Buddhist Insitute/Jiunzan Tendai-ji (and affiliated groups), California Tendai Monastery and the four temples in Hawaii are the seeds of Tendai in America
It seems, that there are ways to practice Tendai in the US, but the scriptural foundation is still unsatisfactory. Apart from the few translations of Tendai teachings, there are not many classic works available in English, as Seishin and mr. gordo rightly pointed out.
I think this is the main problem Tendai has today - because most people in the West (including myself), first encountered Buddhism through reading instead of practising meditation or visiting a certain temple/group. So as long as people only read introductions to Zen and Vajrayana, because Tendai doesn't have those introductory texts, Tendai imho will not flourish in the US.
Zen and Vajrayana have these texts, written by teachers of that traditions, who have an excellent pool of translated texts to study with (and experience with the practice of course, but that is also available for people interested in Tendai) and then write easy commentary/ introductory texts, which fascinate people and make them look for that certain tradition, while Tendai teachers don't have many classic texts to base their introductory books on (but there are also not many introductory books about the practice, which is maybe easier to write at that moment).
There need to be more books and articles about Tendai written by practitioners in English to attract people to Tendai. Today there are only translations, which most people do not read, because they are difficult to read and understand without a certain foreknowledge, and of course academic literature about Tendai, which also isn't very helpful for people who are more interested in Buddhist practice than Buddhology.