Why 11 planes?

I'm sorry if this info is already in the Unicode website or book, but
I searched and couldn't find it in a hurry.

When extending beyond the BMP and the maximum range of 16-bit
codepoints, why was it chosen to go upto 10FFFF and not any more or
less? Wouldn't FFFFF have been the next logical stop beyond FFFF, even
if FFFFFF (or FFFFFFFF) is considered too big? (I mean, I'm not sure
how that extra 64Ki chars [10FFFF minus FFFFF] could be important...)

Thanks.

-- 
Shriramana Sharma