This is happening as there are currently 2 limitations on available tenures: maximum tenure allowed and minimum tenure allowed.
Maximum tenure allowed is calculated on a number of factors, primarily the college graduation date in case of students. Currently, we are not offering tenures that are greater than the remaining months the student is in college.
Minimum tenure allowed is calculated on a number of factors, most prominently the average EMI users are able to repay without defaulting. slice wants to be a responsible company and not offer products on tenures that can't be repaid by the user.
If you are seeing a limited number of tenures available at the time of purchase, it means that other tenures are not allowed for your case due to the above mentioned limitations.