Why are jumbo frames not enabled by default

[Rod_Robertson2]

I understand that certain WAN connection cannot take jumbo frames, though for the rest of the network , why are they not enabled by default ?
What if any are the disadvantages of having Jumbo frames enabled at at the max MTU size 9216 on all ports on all switches.

[Rahman_Duran1]

Enterasys gear has it enabled on all ports bu default as it seems. I don't remember If it is on Extreme gear.

[Bill_Stritzinge]

Rod,

Today there is not a penalty to turning on jumbo frames on all ports, those device that it cannot negotiate a larger MTU it will just negotiate and start talking. My usual base settings for configs are to turn it for all ports all of the time.

Bill