The simple answer is size. Glibc 2.1 increased in size dramatically; apparently this is due to the library introducing support for 64-bit environments like the new upcoming Intel chips.

However, there is in development an Oxygen variant with glibc 2.1.3. There was a significant tradeoff in terms of disk space (150k or more).

The most recent version of glibc 2.1 is 2.1.3.