In comments to Daniel's Pantsu Witches post, Steven den Beste writes:

The reason for the lack of pants is that none of the statuettes had pants, and that’s where the character designs came from.

Quite true. However, where is fun in that? Determining what Toga were doing in the forest when Rakka got stuck is the supreme fun for a thinking fan (of course we know that really it's just ABe screwing the pooch).

Unfortunately, we may be dealing with an exception here. SDB is known to be the master of ret-con and have even done some work on Strike Witches in particular. But the No-Pantsu Problem, evidently, exceeds any possibility of folding it into a rational universe. All we can do is to face it.

UPDATE: Steven says, that TVTropes people did try:

Among other things, they attempt to retcon the pantslessness. One explanation: the Strike Witches can't wear pants because they get in the way of wearing the Striker Units. And the Strike Witches have become famous and are idolized by girls all over the world, who have started imitating the pantslessness. I don't buy it, but it's a nice try.

Indeed, I don't buy amended school uniform code.