So, can you point me to some research, what causes the relative frequency of consonants in various languages?
The fact that vowels are more common than consonants is obviously caused by phonotactics, but I don't see a simple explanation for the fact that some consonants appear to be way more frequent than others. For much of my research, I simply assumed that most of it is caused by syntax, but, evidently, syntax plays only a minor role. As I've explained on this web-page, it's relatively easy to measure the effect syntax has on relative frequency of consonants.
To summarize the relevant part of the web-page, I made a simple computer program in C (source code is available on the web-page) that randomly picks two consonants from a text-file a million times, and counts how many times the two consonants happened to be the same. If you run it on a long English text, it will print that the probability of choosing the same consonant two times in a row is 1/11, and that the most common consonant is t (presumably because of the words like the and that). However, if you run it on an English word-list for a spell-checker, it will print that that probability drops to 1/13, and that the most common consonant is r (probably because of the common English prefix re- and the common English suffix -er). Similarly, if you run it on a long Croatian text, it will print that the probability of choosing two same consonants in a row is 1/13, and, if you run it on a Croatian word-list, the probability will be 1/14 (in both cases, the most common consonant will be n, probably because ne- and na- are very common prefixes forming Croatian words). And, if you run it on a long German text, it will print that the probability of choosing the same consonant two times in a row is 1/12, and that the probability of that happening in a spell-checker word-list is 1/15. In both cases, the most common consonant is n, and I can't really guess why.
So, as you can see from the above data, while syntax indeed plays some role in the relative frequency of consonants, that's not all there is to it. To what extent is the rest of the effect caused by phonology, and to what extent is it caused by morphology?