Several reasons:
1. Those were the tubes that came with the wheel - with 42 mm valves stems.
2. I still have a stock of 42s for my other wheelset with 23 mm rims.
3. I like the aesthetics of having the smooth rim profile - undisturbed by a poking valve stem. But that's just me.

However I did already reserve a couple of 60 mm-stemmed tubes from a bike supply store.