This is happening because there are 2 limitations on available tenures - maximum tenure allowed and minimum tenure allowed.
Maximum tenure allowed is calculated on a number of factors, like order amount or 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, then that means other tenures are not allowed for your case because of the above 2 limits mentioned.