Honestly?
There are many reasons one being the one springfield mentioned.
Some / most / single threads often get stuck at a specific point and then there's just a higher chance of doing it over again and getting it solved properly.
But the biggest point IMHO is for SEO, if you have more content on your website search engines will index more of it - if a search engine indexes a page of yours the possibility is again higher of you getting a visit - and this will automatically maximize your exposure to the WWW.
I myself did near to none SEO optimizations for UGBASE tbh we "only focus" on SA:MP there's not much We could advertise for.
Oh... and probably because a thread died for a reason may it be for reasons like there's no solution to the problem / no answer to a question or the users discussion in these thread being inactive.