[aravindet]

One more: because its value depends on network effects. For example, it might be a communication service, a community forum or a multiplayer game.

Free (of cost) results in more users, which makes the service more valuable for every user. Unfortunately, it also prevents the developer from capturing this value directly.

In some cases, user segmentation and price discrimination is possible (ancillary paid features) but they come with extra design and development cost.