Why is meat dry in stew?

So long story short I tried to make a beef stew, I used chuck beef cut into 1 inch cubes, potatoes, celery, beef stock and what not, and then left them in a slow cooker for about 6 hours but when it was done the meat was really dry. I was wondering if there was a reason that the beef became like that -for some reason thought that beef in stews couldn't physically be dry because its in water. What do people do to avoid this and get really tender pieces. Sorry if this seems like a dumb question I'm just starting to learn how to cook. Thank you!