Because Feisty is alpha? But I will install it when it's released April 19.
The whole purpose of this thread however, is to compile your own new kernel from source. I bet even Feisty will lag behind the newest kernels after its release, so it'll still be needed to learn how to do this if you want the newest features or need some new feature to support hardware on your PC.
Energiya: About my problem to run X after compiling the kernel (see above); I wonder if I need to reconfigure and then recompile the kernel after I've changed to inlude autoconfig.h? I'm confused about how to proceed with that problem. It didn't work just to touch the file.