The unix and mac world used the 4 character extensions first... Windows used the 3 characters due to limitations in the filename extensions waaay back in the day.

Since it is HTML (HyperTextMarkupLanguage) it was more popular to use the full 4 character extension.

These days having a *.htm extension is usually a sign of an old website (from when these limitations were in place) and thus instantly "dates" your site as being old.

In fact these days, the trend is to get rid of the extensions entirely! (at least in the URL) and use the web servers mapping (e.g. Mod_rewrite) to map to the real files on the system.

..because they're still using FrontPage 97 ;)

Extensions are dated as a concept when it comes to webpages. .whatever doesn't mean a thing anymore as the http header tells you what the file is, and how to interperate it.

I personally hate any site that uses .asp .aspx .php .jsp .cgi .htm .cfm .py .etc as in todays world it means nothing, also don't understand why you would want to advertise what language your using for backend workings.

..because they're still using FrontPage 97 ;) +100
or they are sttupids

I disagree that using a *.htm extension dates your site. First of all, most people don't know the difference. Second of all, I personally always use *.htm, and I know I'm not the only one. Use whichever you like. I doubt it makes a difference.

However, as Scunliffe pointed out, using fancy ways to get even nicer looking URLs is often an even better option.