Post his comeback from injury, Ishant Sharma had struggled to gain his rhythm and perform in the New Zealand test series. With age not on his side now (he’s 33 currently), I think the management has moved on to younger fast bowlers like Siraj and Prasidh. This transition had to happen at some point in time and there is ample evidence it is taking place now.
He has been demoted from the central contracts to the B category. He has refused to play in the Ranji trophy, jeopardizing his chances of selection further. For him to come back into the team, I believe Ishant needs to perform at the domestic level again.
In the case of Umesh Yadav, I think it is unfortunate that he is not selected. He had played 1 match in the South Africa test series but picked up only 2 wickets. He had a great IPL wherein he picked up 16 wickets. With age not being on his side now (34), I think he is facing a similar fate as Ishant Sharma
The only positive in Yadav’s case is that he has signed up for Middlesex to play county cricket. I hope he can make a strong comeback to the team.