could you tell me why? because the other ones have more cores and more caches
Biggest reason would be that your motherboard doesn't support the FX series of processors. Other then that depending on what you do the 6100 isn't really faster than the Phenom II 965. And technically the fastest processor supported on your board is the 945, 960T, or 1055T.
Supported CPU list also lists the minimum required BIOS for each chip -> http://www.asrock.com/mb/cpu.asp?Model=N68C-S%20UCC
Your motherboard does not support 125W chips which means the Phenom II 965 is not supported.