You'll note, once again,the difference between today's street level, and the level in Roman times. The Forum was a swamp in between. More than 2,500 years ago they drained the swamp. That sewer, the Cloaca Maxima, works well today. Numerous and successive barbarian invasions depopulate and devastate the city.
Malaria becomes rife.
The city effectively dies. The forum, and much else on low ground in Rome, begins to return to its pre-civilization state. When building a new structure it was often easier to create a foundation by filling the walls of the previous structure on the site with concrete and building upwards. And so one finds the Piranesi etching showing cows grazing on top of the Forum, with only a column or two sticking up from below. For any number of reasons, ancient cities grew up , literally.