Even in the age of 160GB disk drives, very few of them are shipped from the manufacturer with BSD on them -- they have to be loaded from some media or another. The most common current choice is a CD-ROM which is fixed at ~600MB.

Having spend a small chunk of my career assembling OS distributions for dissemination: I can tell you that having an entire, working OS on the smallest whole-number of CD's possible is important. And on a 600MB CD, 26MB for a scripting language is a pretty hefty footprint.