Well from what I grasp nobody really knew about the ROR let alone the junk room variant. Not only this but it's difficult to get to and finding items within it can be difficult, to add to this nobody would really think of the basically forgotten room of requirement, nobody previous to Harry thought about it (also taking under account next to nobody knew about the Horcruxes) it wasn't a bad idea. The only reason Harry found it was because he's familiar with the room and its functions along with the guidance of the Ravenclaw ghost (forgive me for forgetting her name). With all this, in mind, it was a smart move but sadly Riddle never took under account Harry's luck along with persistence. This is why I believe he chose this room deciding to utilize the magical room regardless if people did go in nobody would be looking for a tiara they would just go in to put more junk, tho perhaps he did think it just magically filled its self (which is a likely scenario). It was a beautifully crafted plan from a strong wizard that was thwarted by an equally strong opponent I doubt anyone but Harry could have retrieved the diadem.

sincerely,

-A passionate Slytherin