Some times i ask my self . Why theravada reject such important teaching of buddha !
In the very first place, Theravada does neither rejection nor acceptance. Theravadins, the followers, however does what you're lamenting about.
Secondly, being a specific surviving ancient school with their own specific texts in teaching & discipline, they're not obliged to accept anything from anyone else what they do not regard as 'Buddha vacana', be it from their previous other ancient rivals back then or contemporary ones of today. So what is 'important' to you may not be important to them. And they're not obliged to keep fitting into what others fancy about them... The same is true for the vice versa
Thirdly, the closest that some Theravadins will accommodate as Buddha vacana from others may be in the spirit of this passage
...And the Blessed One spoke, saying:
"In whatsoever Dhamma and Discipline, Subhadda, there is not found the Noble Eightfold Path, neither is there found a true ascetic of the first, second, third, or fourth degree of saintliness. But in whatsoever Dhamma and Discipline there is found the Noble Eightfold Path, there is found a true ascetic of the first, second, third, and fourth degrees of saintliness.
Now in this Dhamma and Discipline, Subhadda, is found the Noble Eightfold Path; and in it alone are also found true ascetics of the first, second, third, and fourth degrees of saintliness. Devoid of true ascetics are the systems of other teachers.
But if, Subhadda, the bhikkhus live righteously, the world will not be destitute of Arahants."
So, that's why we can see today contemporary comparative studies and discussions in Vinaya & Abhidharma issues & studies and great interest in the Agama Sutras compared with the Pali Canon. Then we have another group of Theravadins who are willing to engage beyond these like the late Ven Dr K Sri Dhammananda Maha Thera coming up with his version of the Trikaya in his seminal work of 'What Buddhists Believe' and the late Ven Dr Walpola Rahula who has an opinion on the subject on the alaya vijnana & both having musings on the Bodhisattva Path. But even so, that does not mean that all Theravadins agree with these two either nor are they obliged to...
All religions sects they have some difference in the teaching but not reject the important teaching of their founder .
Again, what is 'important' to you may not be 'important' to others. There are similarities that both sides share and can agree on as commonly held ones which have been repeated to the point of ad nauseum on this forum which I will not regurgitate again.
Like pure land teaching is most important teaching of buddha . Why theravada reject such teaching !
Again & again, what is 'important' to you may not be 'important' to others. They have the teaching and practice on buddhanusati but it's not the same format and extent that the Mahayana have developed. The sooner I accept this the better I can move on not insisting that the Theravada and Theravadins should or must accept the same thing as my own Buddhist teaching & practice or tradition. This is like insisting that all others must look or think the same as I do just because I think so...