Tsunade slacking off!

[Raidenuser]

Kakashi hasn't got any Spare Chakra to Seal away for emergency.

Kakashi's Chakra Levels are low Enough as it is without Him sealing apart of it Away.

Kakashi is like always on a mission...

Well that is what training is for.

A combination of these ^^ not everyone is suitable for medical technique so it is possible that this is one jutsu that Kakashi can't pick up quickly and it must be high level or everyone would use it

Also he seems to be one of the hardest working Ninja's - he is constantly training or on missions, he needs all the chakra he has

 

[EliteKakashi]

And it seems to me that Kakashi is doing fine with his chakra throughout this war. Not sure he really needs something like this.