I want to know reason that why Lord Krishna was not worthy for syamantaka jewel?
The Viṣṇu Purāṇa itself gives the reason:
But this jewel, to be of advantage to the whole kingdom, should be taken charge of by a person who leads a life of perpetual continence: if worn by an impure individual, it will be the cause of his death.
Now as I have sixteen thousand wives, I am not qualified to have the care of it. It is not likely that Satyabhāmā will agree to the conditions that would entitle her to the possession of the jewel; and as to Balabhadra, he is too much addicted to wine and the pleasures of sense to lead a life of self-denial.