Because you can't buy a "restricted" one-way fare on this route. Believe it or not but your quoted £3600 fare will have restrictions - it won't be a full fare ticket.
I'm assuming this is a Club World return price?
If you can see redemption space for the MIA-LHR sector and you want to go ahead with that my suggestion would be to book it. Then book mixed class JFK return (out in Club and returning in Economy handbaggage only) and simply don't show up for the return sector (suggest booking a different return date all together to the MIA-LHR)
To give you an example using your outbound date:
LHR-JFK 13MAR Club World
JFK-LHR 27MAR World Traveller HBO