I won't speak to those people specifically, but different reasons include
- Different elite benefits... DL and NW may be in the same alliance, but they don't offer reciprocal upgrades.. if you fly both due to their route structures you may want to be upgraded with both.
- Different partners... you may want to accrue miles with whomever offers the better deal with a partner, or via a partner with one carrier that doesn't participate with the other.
- Different bonuses... I recently credited flights to a new AZ account to take advantage of their anniversary bonus offer, rather than my existing DL/NW/CO accounts.
And several other reasons...