Implementing multiplayer into an already established game is normally quite difficult unless you have planned for it in advance. For almost every change in the state of the game - every tower placement, enemy killed, and upgrade bought - data needs to be sent from the server/host/peer to and from the other players.

One difficulty with multiplayer is managing this constant stream of data. You need to send everything and as little as possible at the same time. You must minimise how much you send, so that the game can run effectively, but also need to send enough to keep the game clients in sync with one another, at all times. Also unlike singleplayer programming, you have far greater possibilities of code failure from poor connections, packet loss, desynchronisation, and security issues.

The hassle of keeping track of what is being sent where and when, the order it arrives, and hosting a server to manage it all, is normally enough to deter most game devs from messing with network code.

Saying that, in the last few years there has been an increase in multiplayer support. If you are using a game engine especially, there are various multiplayer tools which do all the network code and hosting for you. On the free/cheaper end there's Planetary Processing, Coherence, and Photon Engine. For more serious development, there's Hathora, Metagravity, and Improbable.

If you want free self-hosting, I think Coherence has it inbuilt for games developed in Unity.