Full sponsorship (no fees) including travel
Transportation to/from Practice (Buses)
Affiliation with a profession team
Multiple full kits and practice uniforms from above professional team including matching boots (cleats)
"Top" coaches, trainers, strength and conditioning, etc
Training at a newer turf field with lights
International tournament play
Game tickets and player appearances and training with.
Yes this is real no joke and was offered at U9 of course they ran out of $ after the first season, the buses never materialized, and had to play in a league that no longer exists (LA Futbol League) since the pro Affiliation what not accepted at the time. Times have changed a bunch since these days but somethings haven't.
When you players has a desire to play at a higher level than your current club can offer (Might only have lower level teams or not in your players age group, or not in certain leagues, tourneys, whatever )
Better coaching. Sometimes coaches move on, get promoted, move out of state, get married, etc and the replacement might not be at or close to the same level.
Coaching is the biggest determining factor . What's their history, licenses, What have they done with their past teams? These are questions to to ask, don't fall for the "come play for us because we are the biggest & best club", the coach needs to answer how they are going to make the players better. Ask the coach and other parents multiple questions.
Opportunities for advancement, and to be seen? what's the track record of the coaches and club in this age, history of college placement for example if that's what your player is aiming for.
Transportation, travel, and amount of time devoted to "soccer" can be a factor also. School work priority, multiple sport playing, travel, etc can be good reasons to make a change.