Learn English – Pronunciation of ‘cos’ (as in the mathematical term)

Just guessing, but perhaps the /ju/, which came by breaking a high front rounded [y] in words borrowed from French, came to be regarded as a high prestige form. Then, hearing /u/, some English speakers interpreted that as a mistaken or low-class way of saying /ju/ and decided to correct it in their own pronunciations. If that's right, the /ju/ from earlier /u/ should turn up in words most easily interpreted as having been borrowed from French. Folk loan phonology.

Take a look at this chart.

I will reproduce the relevant section of it.

. red raid RP e~ɛ ɛɪ AmE ɛ eɪ~e ScE ɛ e AusE e æɪ

So in Southern British English, the distinction between red and raid is that the vowel of raid is a diphthong (and is longer), and /e/ and /eɪ/ are reasonable symbols for these vowels. In Scottish English, the distinction between red and raid is that the vowel of red is more open, and the most reasonable symbols for them are /ɛ/ and /e/.

In American English, raid is distinguished from red both by being a diphthong and by being closer. In fact, some American dialects pronounce this phoneme as a monophthong if it's before a voiceless consonants (as in rate). The phoneme /e/ from other languages is generally heard as the phoneme /eɪ/ by Americans, so the open/closeness seems more important in recognizing the phoneme than whether or not it's a diphthong.

So as an answer for your question: red might be pronounced as /red/ in RP (standard British English) and in Australian English, but it is not in American English and Scottish English.