Another reason why CITR is seen as part of "The Canon" is because the post WW1 white, male writers from the US were so distinctly different from anything that had gone before. Until then, even American writers had the same style as European ones.
As much as CITR can/should be criticized for be a white-boy whine (and it's a valid discussion imo) it was actually part of a new style of literature that altered things such as sentence structure, plot development, subject matter, narrative style etc. Ironically, it's a piece of white male literature that helped to open the doors to a new, different form of writing, and actually played a part in the (reluctant) acceptance of a broader range of writers within 'The Canon".
Achebe's Things Fall Apart was published 7 years later. If newer, less traditional writers had not already been accepted & established, then it's possible that Achebe would never have had the impact he did.
Of course, it's still incredibly difficult for African writers to be published. There are some with international success, and even then, their books are rarely easily available in Africa. So I don't think any aspiring white male novelists need to worry about being shut out of the publishing world just yet.