Why do most people think that the Samurai were conservative,refusing to adopt European weaponry?

The part you are missing is that the Japanese at a point in their culture actually went backwards and removed guns from their society to the point that when Europeans returned to Japan in the 1800s after being kicked out for the most part in the 1600s it looked to the Europeans that the Japanese were not well versed in firearms use. The Japanese had to start buying and making for themselves new modern firearms all over again to make up for lost time. (they did this amazingly fast) You should find a used copy of the book called>> Giving up the Gun Japans Reversion to the Sword 1543-1879 by Noel Perrin