Why does the post anwer become locked after a while?

I think they lock automatically after a week. I really do not know why, it is something the site developer decided upon.

Maybe they lock under other condtions as well. I do not know.

If you want to continue an old thread you can write to a moderator ands ask them to unlock it for you. 

You would need to include the address of the thread in your message of course.

I certainly have done that for people. 

I am not sure how long a recently unlocked question would stay unlocked for though.