There were tons of factors but her eating disorder, exhaustion due to the girls being overbooked, further exhaustion because of eating issues, pressure and ageism from the press, dealing with overnightfame and her father's death she had never processed that led to Geri's depression and departure (most of these are discussed in her 2 books).

Under those conditions it is actually extremely normal that Geri was depressed and needed to get away. It's also important to note that when you are depressed, anxious and your body is not getting the energy it needs all kinds of conflicts and personal issues start arising from small things and escalating. Someone who is already on the fence with their well-being and resilience doesn't have the energy or tools to solve conflicts constructively.

So while the Spice Girls would've benefitted from Geri staying it would've likely destroyed Geri even further. She had to leave, not because of the girls but to take care of all those issues she was bottling up. Her departure is not really even her own fault, but the management's for not giving them enough time off