When I picked up my first mirrorless in 2011, the primary motivation was size and weight - I wanted a second body alongside my DSLR with the same sized sensor but much lighter and smaller for travel and portability.
As the bodies improved with each generation, additional motivations started to take over - ability to adapt other mount lenses was fun, good EVFs became a nice addition, no MFA needed became very convenient, addition of OSPDAF made them more usable for a wider range of photography and started to push my mirrorless system from second body to primary body, and as OSPDAF got much faster and more advanced, eventually mirrorless became a preferred system for all of my shooting, including wildlife and BIF where they previously trailed. New and better lenses continued to come along also adding to the performance and versatility of the system.
Now, I am mostly shooting with a mirrorless system because I find it to be the best camera system for all of my needs. Size and weight are quite a ways down the list of important factors, though still in the mix, since the mirrorless system acts like a modular tool that can shoot with big, long lenses, fast primes, or compact travel zooms and kits or small primes, covering all bases from lightweight travel cam to full-on wildlife rig.