Board Thread:Tapped Out Suggestions/@comment-201.216.165.162-20150515153526/@comment-167.61.1.213-20150515164129

Most likely is due to you not having started the quest associated with them. In addition to reach a certain level, a quest must be triggered for the buildings to be available to purchase. For level 51 The Ungrateful Dead is the main questline, my advice is that you click on that link and start checking the preceeding ones in the infobox. In order to have that questline triggered you need the preceeding ones completed.

The reason of why that happened could be either you leveled up too fast (can happen if you have too many buildings, that makes the collection of money and especifically XP higher that the normal), or you didn't put too much attention to some quest or they were too long and that affected all chain of questlines.