Why are there so many restrictions on /ŋ/ in English?

Question

In (GA, SSBE) English, the phoneme /ŋ/ (in ring) seems to have so many restrictions:

• it rarely occurs after /u:/, if at all: the only word that I have been able to find in which /ŋ/ occurs after /u:/ is mung/moong
• rarely occurs after /ʊ/: as in loong, boong and occurs instead of /ʌ/ in dialects where /ʌ/ is realized as [ʊ] (mainly Northern British)
• /ŋ/ never occurs in the start of a syllable in English
• rarely (or never?) occurs after /aɪ/
• rarely (or never?) occurs after /aʊ/
• rarely occurs after /eɪ/
• rarely (or never?) occurs after /ɔɪ/
• almost never occurs after /əʊ/

Another phoneme that has many restrictions is /ʒ/ but I assume it can be attributed to the fact that /ʒ/ is not a native phoneme and I am not interested in that phoneme. (There is also /h/ with many restrictions, but I am not interested in that either.)

Is there any reason why /ŋ/ has so many restrictions?

Answer 1

Most of the unusual behavior of [ŋ] can be explained in its historical origin from simplification of the consonant cluster [ŋg] (in words where [ŋ] occurs in modern English outside of the clusters [ŋg] and [ŋk]).

Compare the behavior of the consonant cluster [mp]: like [ŋ], [mp] cannot occur at the start of a syllable and rarely occurs after a diphthong or "tense" vowel.

The clusters [nd] and [nt] can be found after diphthongs more frequently than [mp] or [ŋ], but that is part of a general pattern where syllables ending in coronal consonants such as [d], [t], [s], [z] are allowed to have more complicated structure than syllables ending in non-coronal consonants.