My guess is that the selectors want them to play Ranji Trophy (4-day domestic competition in India) instead of the T20 series against Sri Lanka. They want to make sure that these players remain 100% fit for the longer format. For the shorter format, they have so many options (Shardul, Saini, Khaleel.etc), but that is not the case with the test format.
I have to disagree here. In test matches, it is the wicket taking ability that matters. So the selectors may prefer bowlers with better strike rate, such as Umesh Yadav and Ishant Sharma. On the other hand, in the T20 matches discipline counts a lot. Bowlers with average speed in excess of 140 kmph may find themselves at a disadvantage in T20 format and it is usually the medium pacers who are preferred there. But I agree that the spin bowlers don't have such limitations.