The real question is, "Why doesn't Ruger do a better job of explaining the reassembly process in their manual?" Seems to me that they could simply change their user manual and greatly improve customer satisfaction with their Mk II product. A simple, inexpensive thing that they've simply neglected to do.
Sounds like some of you guys haven't really learned how to reassemble the Ruger Mk II. (That sounds like a smart-a$$ comment, but I don't mean it that way.)
I had a Mk II Target Competition (Slabside) for a number of years, and once I finally learned how to reassemble it, I could do it in a minute or less, with no problems.
It all has to do with that little "dangle" hanging down inside the grip, where the mainspring slides in. (I think its called the hammer strut.) Get that lined up right, and its a piece of cake. And getting it lined up is more a matter of paying attention to what you're doing than anything else.
And, when you push the mainspring back in, if its flush with the grip, you've not positioned the strut properly, so pull the spring back out and try again.
The first couple of times I stripped it, before I learned the secret, getting it back together was a real hit-and-miss proposition.
Try this site for great instructions -- with pictures:
http://www.ontargetguns.com/rtips.html
(I've since traded the gun for a match-grade .45 -- but I found it MUCH easier to disassemble and reassemble than my custom 1911A1 with a full-length guide rod.)