A very large portion of knives are tip up only, especially framelocks and flippers. There are a few (mostly Spyderco) that have four way clip position. Tip down just:
A: Is not very popular, especially with "tactical" and waved knives
B: Gets in the way while trying to use the flipper
C: Depending on the design, it would get in the way of the lock release.
If tip down is absolutely necessary, your best bets are:
ZT 0350 and 561
Spyderco Military and Gayle Bradley
If a flipper is important to you, Kizer has quite a few tip down flippers.