When a project is developed for a while, a lot of initial design decisions can become invalidated as business needs evolve. New features have to be added, and in many cases they go against original assumptions about how the project would be used. At that point you have to start making hacks and kludging new features in. This creates a lot of special cases and surprising behaviors making overall project brittle and hard to maintain. That's what's known as tech debt.

In an ideal world you would have time to do proper redesign to accommodate new features, clean up problems as you go, and so on. However, in reality there's usually just not enough time to do any of that so people just pile on features at the cost of overall development becoming harder and more error prone. This is a great discussion on the subject incidentally https://medium.com/@wm/the-generation-ship-model-of-software-development-5ef89a74854b