How is Sherlock Holmes work SCIENTIFIC?

Answer 1

Simply because it involves forensics. When the minutiae mattered and a single detail could prove or disprove a man's alibi, the knowlege of, say, how long bruises can remain post-mortem became invaluable to Holmes. Sherlock Holmes knew how to identify poisons, what bit of dirt was from where by color and consistancy and whether a drop of water came from the Thames or from Niagra Falls. Such knowlege was very useful to him and without it he wouldn't have been able to solve some cases simply from lack of information, wouldn't be able to "make bricks without clay", as it were.