"Regular" fullscreen involves taking "exclusive" access of the GPU, which means a lot more work in handling resolution switching, resource management (in older APIs, in particular) and so on, especially around supporting correct alt-tab behavior. This might seem odd at first, but it arises because when you lose exclusivity (when alt-tabbed out), your game's GPU resources are effectively evicted and must be restored when you tab back in.
A "borderless fullscreen" window is just a regular window, with window chrome disabled, that is the size of the whole screen. This means it acts just like a regular window and doesn't incur the extra complication and overhead of exclusive GPU access, which makes things easier for developers at the cost of some negligible performance. This is the main technical reason: less work.
It also makes things easier on users, usually, because many games that use exclusive fullscreen modes also happen to not handle alt-tab very well, and crash, or act oddly when restored to fullscreen. This can be exceedingly annoying for players who want to alt-tab to look at GameFAQs, chat on Discord or IRC, or whatever. It's also more friendly to multi-monitor setups.