This has to do with the order the pages are set to display by default,
it usually is set like index.html, index.htm, index.php, etc…
If for instance you install a wordpress site, it would be using the index.php but it would never get displayed as long as the default “under construction” page is still there.
As stated above, rename the index.html to something else to get it out of the way temporarily and that should solve your problem.
You can also change the default order and move the index.html down to the end and achieve the same thing that way if your php page stopped working for some reason the other pages would pick up still.