Why is Juliet isolated?

You've noticed, probably, that the only reason Juliet has a chance to get out of her house is when she goes to church. This does indeed make her isolated and quite incapable of dealing with the outside world, which might explain why she does not take her father up on his offer to let her "hang, beg, starve, die in the streets". She doesn't have to do any of these things, of course, because she can go live with Romeo, but she has no knowledge of how to live on her own.

This cloistered treatment of young women may have a basis in the customary treatment of girls in medieval Italy--it certainly is accurate for Spanish custom. The English were much more liberal in their treatment of girls and women, to the documented surprise (and grudging admiration) of continental visitors.