it is because that most of the people buy .com and about 60% SUCCESSFUL domains are .com on the internet.
ummmmmmmmm .coms nd .orgs are kinda given preference i guess.. cuz thtose are mostly the result i get through google. but it kinda depends on the site popularity too
in the eye of google, all LTDs are treated the same but for visitor the easier and common LTD to remember the better. that's why most webmaster prefer .com and mainly webmaster regard otehr TLD as the alternative or not important so they dont want to develop those to rank high.