Howard will have to provide the definitive answer but a couple of thoughts come to mind.
1. It eliminates thread bumping in the classifieds. (Everywhere actually, but it's most annoying there)
2. Reducing the total number of posts will make the databases & searches faster.
3. If you're the only one posting then sniping a post number is a bit of a dubious achievement at best.
Thanks TC - had not thought about thread bumping in the classifieds.