This is happening because, default behaviour of
struts 2 is lookup for an entry in struts.xml when,
1. requested url has extension .action
2. or there is no extension at all.
Since request made on context root does not have any extension your page opens.
But, now you enforsed struts to lookup only when extension is .html hence its not working.
Do you have any good reason for not to use web.xml's welome-file-list