Because no one knew it was there. It took Voldemort almost half a year to simply find out that Dumbledore was the last wielder. Even after Voldemort's death, Harry just fixes his holly and phoenix wand and puts the Elder Wand back in the tomb.



Everyone would expect something that powerful to be locked away in a vault or other hard-to-access place, but no one will bother or even think of checking a much less secure area, because only a crazy person would leave something so valuable in a place such as the tomb of Albus Dumbledore, who isn't even the master of the Elder Wand anymore as far as any ambituous duelers would be concerned. Harry publicly announced in his last duel against Voldemort, in the Great Hall, that if he was the master of the wand, a fact that was proven when it refused to kill him and let Voldy get blasted by his own spell for the second (and last) time. If anyone were interested in it, Harry would be the person they'd be looking into, not the tomb of a revered headmaster whose only remaining link to the wand is that he was the last wielder before Voldemort and Harry.



I mean, there's no hint that the wand's been stolen in the 19 years after, so it looks like so far it was crazy enough to work. Plus the only people who know of the Elder Wand's location are Harry, the pre-Snape headmaster portraits who listened in to Harry and Dumbledore's conversation, and possibly Ron and Hermione (I don't think it'd be unlikely if he trusted them with this information).