How come rude is not pronounced as /rjuːd/?

It used to; the [j] was regularly dropped after certain consonants:

Phonological history of English consonant clusters – Wikipedia

The change of [ɪ] to [j] in these positions (as described above) produced some clusters which would have been difficult or impossible to pronounce; this led to what John Wells calls Early Yod Dropping, in which the [j] was elided in the following environments:

• After /ʃ, tʃ, dʒ/, for example chute /ʃuːt/, chew /tʃuː/, juice /dʒuːs/
• After /j/, for example yew /juː/ (compare [jɪʊ̯] in some conservative dialects)
• After /r/, for example rude /ruːd/
• After stop+/l/ clusters, for example blue /bluː/

Apparently Welsh and some other dialects kept [ju] as [ɪu], did not undergo yod dropping, and as a result they pronounce chews /tʃɪʊ̯z/ and choose /tʃuːz/ differently. I can’t tell from Wikipedia whether that extends to rude being pronounced as ree-ood.