The 'official" party line from Peter Jackson and New Line has always been that very early on they realized that they had cast Aragorn "too young"...

That could mean that ST LOOKED too young to play Strider... or it could mean that he was ACTING like a spoiled little sh!t-stain and quit before he got fired...

I suspect it's a little of both...

As unimpressed with Viggo as I am, (Method actors are navel-gazing idiots!) I still count Stuart as a bullet dodged...