Interest under section 244A

Why was the refund credited late?   

Generally, interest till the assessment is granted.

As per the provisions of sec. 244A i.e. interest on refunds, interest is payable from 1st day of assessment year to the date on which refund is granted at 0.5% for every month or part of a month if return is filed under sec. 139(1) by the department. 

The interest is  payable at 0.5% for every month or part of a month from the date of furnishing of return to the date of which refund is granted if return is filed other than sec. 139(1).

In this case refund is granted in March 2019, so interest will be payable till march 2019.