This is an interesting phenomenon in language change. There is the question of what sounds are legal in the language (or what sound combinations, and in what contexts), and there's the separate question of which of those possibilities actually exist. For example, syllable-initial "th" is never followed by L in English, but the sound sequence generally doesn't pose difficulties to English speakers, who are already familiar with words like e.g. "three", "shrink", and "sleep".
For a sound to occur in a language, there are two requirements:
The obvious one is that it has to be possible.
But the less-obvious one is that its precursor in the ancestral language must have been possible there. This is what causes strange gaps where it looks like a sound should be possible, and yet it never occurs.