"Since I am a writer and not scholar and can make connections with little proof, this all sounds like a conspiracy to keep a good man down. *shrugging my shoulders*
I know this is all speculation, but was Brasidas too audacious for conservative Sparta? " (Brasidas Xanthippos)

I too agree that he may have been purposely kept down by fear of gaining too much power/recongnition. Spartan society shunned individuality, but Brasidas had qualities that made him shine above almost every Spartan before him:

"Brasidas united in himself the personal courage characteristic of Sparta with those virtues in which the typical Spartan was most signally lacking. He was quick in forming his plans and carried them out without delay or hesitation. With an oratorical power rare amongst the Lacedaemonians he combined a conciliatory manner which everywhere won friends for himself and for Sparta." (Thuc. iv. 81)

Any elder within the Spartan political system could recognize that such qualities and boldness would clash with their tradition of modesty and collectivity. Hence, out of conservatism, they may have sought to keep him from rising in power.

Again, this is just a speculation. And it does nothing to answer why the Spartan elders would be so quick to supress Brasidas, but did little to keep Lysander from ascending to his position.