Tai Tzu is rather common in China, and Taiwan, just not in the US. Even that being said, there is more of it here than meets the eye.
Tai Tzu was spread VERY prolifically through the military during the Ming dynasty. It was also spread by the Chao family, and the Shaolin temple during the Sung dynasty.
Because of this, there were many, many lines. Tai Tzu is probably the widest spread Chinese art there is. Because of this, complete Tai Tzu lines vary in size and curriculem, and each has it's own evolution. Some branches today are just a single form. Many are three forms, or one form taught in 3 sections. Some are 10 or 12 sets, others still are 2 forms, the original 32, and one really HUGE set that is 18 sections long (common in Hubei i believe)
All branches are very similar despite the size though. You can see by the flavor,and the presence of several, to all of the original 32 techniques in every form. You can't just do a modern Chin Woo form, and call it Tai Tzu because both are Long fist. Tai Tzu has distinct techniques and flavor found in every form. You also can't do Tai Chi, and call it Tai Tzu. The arts are too different.