What stops an assistant/associate professor with a strong publication record from being promoted? As far as I experienced, research is the most important factor for promotion, as it represents different features of an academician/scholar, which are needed by higher education institutions: teaching at graduate level, supervising academic projects of graduate students, attracting research fund, fame for the university, etc.

Publications is normally the main measure for research of a professor. Thus, one expects to see a proportional relationship between publications (both quality and quantity) and academic rank.

People can be promoted with less publications, probably because of other activities. But, I wonder why there are some academicians whose rank is far behind their publications. For example, I have seen people with 50 papers but are still assistant professor, or over 100 papers but still associate professor. I am referring to the US universities.

What keeps an assistant/associate professor with a strong publication record from appropriate promotion?
Remember, a promotion/tenure review package is just that: a package. While you are correct that a record of strong publications is the driving factor, there are still a number of items in the package that could lead to a tenure/promotion denial:


Weak letters of recommendation. You need to get your name out in more ways than just cranking out a lot of papers. This includes presenting talks at other institutions, sitting on program committees, and making sure that prominent people outside your institution write you good letters for your package.
Lots of publications ≠ good publications. You need to be published in reputable, peer-reviewed publications. Some of those professors may list a ton of publications that aren't particularly notable.
Poor record of graduate student supervision. A professor who never has any students, or who hasn't graduated a student in years isn't going to be competitive for promotion.
Poor fundraising. If you're not applying for and getting grants (in many fields), you aren't going to be able to sustain a good research lab. It's tough to keep publishing relevant work without money coming in, although over time you can still build up a fair number of publications (many of which may be mediocre). 
Poor teaching record. At some institutions, good teaching is more than just something the admissions office declares in all of its trifolds. If you've had consistently poor teaching evaluations, or if students have complained multiple times to the Dean or department chair, this could hurt your chance at promotion.
Personality. Your department has to make a recommendation to the tenure/promotion committee, and if you're genuinely not liked around your department, they aren't likely to make that recommendation, regardless of how strong your publications are (within reason).


Bottom line: publications are a part of the larger promotion picture in academia.