A couple of days ago I was discussing the semi-permeable membrane between "high" and "pop" culture with someone, and he brought up Tolkien. He restated the conventional wisdom that the reason it took so long for Tolkien to be taken seriously was because of snobbery about fantasy.
Very likely snobbery had something to do with it, but I think there might have been other, more influential reasons. The primary reason, I think, is simply that medieval scholars felt restricted from leaving their field.
Until the era of "medievalism," in which modern representations of the medieval came to be considered fair game for medieval scholars, Tolkien's fiction was not a proper subject for a medieval scholar to study -- not because he was producing fantasy, but because he clearly falls into the modern era. A quick perusal of the MLA areas will reveal that even today the study of literature is balkanized by period to some degree. A medieval scholar who wanted to get tenure or promotion needed to produce serious scholarship, not play dilettante in the modern era.
OK, you might say, fair enough ... but then why didn't the modern scholars do more work on Tolkien? Why didn't specialists in 20th century British literature canonize him?
The problem is that Tolkien's fiction taps into a deep well of philology that almost no modernist can dip into. How many modernists have a working knowledge of Old English? Old Norse? Any of the other medieval Germanic languages? How many know what the futhark is, and how it differs from the futhorc? How many understand what Tolkien means by the wapentake, and how Tolkien's use of the term indicates what he thinks it must have meant?
Of course, some modernists have these skills, but not many ... and why should they? In this regard, Tolkien is unique; I can't think of another single modern author for whom a deep understanding of medieval language or culture is a prerequisite to serious study. Most modernists would have difficulty understanding the rich foundation of learning upon which Tolkien's fiction rests, and those that do understand it aren't enough to make produce the critical mass necessary for canonizing an author.
Don't blame the modernists; blame us medievalists. We took a long time to decide that it was OK for a medievalist to study modern authors. Not everyone can be Tom Shippey.