This is not certainely an excuse to not include them.Most of these enemies don't even spawn often enough for you to kill them 50 times, let alone need a banner to fight them.
I'm not saying it's a reason to exclude them, I was just pointing out that most people don't fight them very often. Also, the martian probe is an interesting enemy type to say the least, seeing as it is the sole way to begin the Martian Madness event.This is not certainely an excuse to not include them.
Before the banner buff in 1.3, the banners only purpose were for decoration.
Therefore, Nymph banner, Rune Wizard Banner and Cultist Banner exists.
My guess is that it's likely an oversight, there are quite a lot of enemies in this game.