Because there is no RNAV Approach for RWY13L.
To be honest JFK is one the absolute worst airports in the Western Hemisphere.
Giving non precision approaches with lead-in tight visual manoevring in marginal weather is an absolute insult to Safety, yet accepted by the FAA and all Operators, including the one I fly for. Luckily, we do train this approach almost every sim.
The whole JFK operation, from approach to taxi into the gate is full of Safety issues, compounded by less than impressive, obnoxious and non standard ATC.