This creates an incentive to severely ramp up the portfolio risk in the hopes of striking it big. Sure, almost all client portfolios will go broke, but the winning one will pay off for the losers.
Such wealth management firm would have an issue surviving past year 1.
Charging a percentage for assets under management creates an incentive to grow that pie, slowly but surely, keeping everybody happy.
Charging a flat or an hourly fee creates an incentive to "wham, bam, thank you mam, next customer please" type of interaction.