Why are only some "u" words pronounced with a glide in American English?

Solution 1:

According to the Longman Pronunciation Dictionary, u: (or "oo") is predominantly used in American English, whereas ju: (yoo) is preferred in British English. However, variations exist, and for the case of student, their data shows that around 90% of American English speakers say u:. (Edited)

On page 850, you can find the following "rules":

In the case of expected ju:, juə, ju, the j drops out as follows:

• after the consonant sounds tʃ, dʒ, ʃ, r, j as in jury, rude

• sometimes in BrE, and always in AmE, after l, θ, s, z as in assume

• usually in AmE, but not in BrE, after t, d, n as in tune