I suppose the reason is that a backplate is not needed for 2011 sockets...see below some information about the spec of the ILM frame of the 2011:
Intel Independent Loading Mechanism
The Socket 2011 employs a new dual-latch design called the Independent Loading Mechanism, aimed to deliver consistent pin pressure across the socket when the processor is held in place. The ILM mechanism consists of:
ILM frame which uses threaded mounting holes for CPU coolers and requires no back plate installation for aftermarket heatsinks.
Load Plate which applies pressure around the top of the CPU
Removable ILM cover which protects the contact pins when there is no CPU installed
Hinge and Load Levers which locks the load plate in place