Why is 'sort of' pronounced /sɔːrdəv/ in AmE though /t/ is not between vowels?

Sort /sɔːrt/

of /əv/

Why is "sort of" pronounced /sɔːrdəv/ in American English even though /t/ is not between the two vowels /r/ & /ə/?

It's not. The phonemic (in the sense underlying) form is /sɔːɹtəv/ which is from combining /sɔːɹt/ and /əv/. Because it is at the end of a word, the /t/ is also at the end of a syllable, and syllable final /t/ after a vowel, a glide, or [ɹ], and before a vowel, in many American English dialects changes to a flap. So the pronunciation is [sɔːɹɾəv]. There is never a d, neither a phoneme /d/ nor a pronunciation [d].

However, a /d/ in this position would also become a flap. So if the phonemic form were /sɔːɹdəv/, it would be pronounced the same way, with flap replacing /d/ (as in "sword of Damocles").