I am trying to make a Bjornager magus, and I don’t understand why some of the general abilities from the Magus don't apply to the Heartbeast? (HoH Mystery Cults page 23)
I can understand why Brawl doesn’t apply to my heartbeast.
But I can't understand why concentration doesn’t; witch is the main reason for the question.
When shapechanged to my Heartbeast I don't have my concentration, but I have all my arcane, academic and supernatural abilities.
But I need my concentration to use magic.
I'd let you have your Concentration. 
Me too. Sometimes rules need to be broken.
I think applying a universal ban on General Abilities is not at all appropriate. I would have preferred a different rule, and IMHO Concentration should definitely be allowed. Indeed, looking at the list I would allow the following General Abilities:
(Area) Lore
Awareness (possibly, not sure about this)
Concentration
Etiquette (in terms of perception and memory)
Folk Ken (possibly)
Leadership (possibly, especially if the Bjornaer had several levels of it)
(Living Language) (understanding)
Music (theoretical)
(Organization) Lore
Survival (in terms of knowing where to find water etc.)
Teaching (if he can communicate with the pupil)
I wouldn't give Awareness nor Survival, both of which are typically part of the beast's skill package. I'm borderline on Leadership. The rest is fine with me.
Thanks for the input.
Maby allow alle the abilities that animals don't have natural.
OR
Use the higher of the two abilities?
Just like characteristics, and virtues/flaws.