There is no cover up.
The Indian Ocean is 26,469,900 square miles covering almost 20% of the Earth's surface. This isn't so bad to search but this problem also extends vertically downwards in an area so remote that the search in itself is as complicated as finding out where to search. At it's deepest the Indian Ocean is 23000ft deep which is just shy of going down vertically 4.5 miles however on average it is around 13000ft which is still around 2.5 miles. They have nailed a search area considerably smaller than that but still it must be painstakingly searched inch by inch in challenging conditions.
A Boeing 777-200ER is 209ft in length with a wingspan of 200ft and could lie anywhere in that expanse in several if not many pieces. Is it any surprise it hasn't been found? They do have an idea of where it went down but that is all, an idea. Searching at such a depth and over such a large area is like finding a needle in a haystack the size of Manchester. Nothing has been found primarily because they not only do not know where it crashed but they were looking in the wrong place at the most crucial time when parts of the wreckage were more than likely floating.
For me it will either be found eventually in the future at some point or quite simply found by chance. It is definitely there but the search will take many years to complete.