Why Tibetan buddhism are not considered as buddhism by other buddhists?
I think you’re misinformed about most of the points you mention. Of course there are some sectarian Buddhist groups that don’t accept certain other Buddhist groups as “true” Buddhists, but my impression is definitely that Tibetan Buddhism is widely accepted among other Buddhist traditions. It might be that many Theravada Buddhists of Southern Asia still don’t accept Tibetan (and many other kinds of Northern Buddhism), but I think that’s mainly because of lack of contact. However, in China Tibetan Buddhism has for centuries had an influence far beyond the Tibetan areas, so the “lack of contact” is not a problem. Actually, Tibetan Buddhism is becoming more and more popular among the Chinese in especially Taiwan and Singapore. The Taipeh Times wrote already ten years ago that there were at least 200 Tibetan Buddhist centres in Taiwan, more than half a million followers of Tibetan Buddhism and over a thousand Tibetan monks invited every year to teach Buddhism to Taiwanese Buddhists:
