Because is track record, writings and opinions, not to mention his own self-descriiption, are such that by and large he is considered to be a conservative by just about every source that is not extremist - the same as Obama is considered to be a liberal.
Few doubted his father's conservative credentials when he broke ranks on occasion and endorsed people like Allard K. Lowenstein and Joe Lieberman. Just as few labeled him a socialist because one of his closest friends on earth was John Kenneth Galbraith.