The site like this are in HTML because they tend to generate pages offline and upload them and the main reason is to have servers when needed does not need to have anything installed but just a webserver. Also I know one ( I cannot disclose ) has such things and I propose them to have user registration facility when I worked with them and they told they cannot do it.
No! Traffic is not prefect reason for using HTML but we can say their style of site suites well on HTML
Don't let the .html extension fool you. Those pages could very well be scripted with PHP. All that is needed is a AddHandler directive in their .htaccess file to treat all .html pages as if they ended in .php This is the same trick I use for a Classic FTP Blogger blog, all of the pages end in .html -- but I also have PHP Includes for my header, sidebar and footer sections in those .html files.
nothing is wrong with html. I wanted to know why is a big company using html and manually editing the site, why dont they use any cms.
It could be just like what shabbirbhimani mentioned above. If this is the case, there is probably some type of database too ... then again, maybe there isn't. No way of knowing without asking them. And again, don't let the .html extension fool you. Take the Yahoo Stores for example. All of the product pages end in .html. Those pages are pseudo-generated from a Product Catalog and uses RTML programming to publish a rendered .html page.