Why do topics get locked?

Earlier today I began a topic with an open question about homonyms.  Glenn posted a comment about an hour later which was informative but not conclusive.  I was hoping to have others to comment, maybe with more information.  But now, about seven hours later, I find that this topic has been locked.  I am just wondering why.  Who decides that a topic should be locked and what is the criteria.  Specifically, why was my topic locked?

I noticed another topic, started by Glenn, was locked earlier and I wondered then why that happened.  But since I started this one and I am very interested in an answer, I thought I would ask if anyone can give some insight into this.

Thanks