Like the Aerospace phenotype before it, the ProtoMech phenotype was determined to be too physically weak to suit the Bears' doctrine of a warrior being as dangerous out of the cockpit as they were in one.
More correctly the ProtoMech phenotype is a development of the Aerospace phenotype, which the Bears did not develop in the same direction as the Clans in general. (The Bears have aerospace bloodlines, they just don't look like those of the other Clans.)
So, back in 3060 when ProtoMechs burst onto the scene the Bears took a look, found they didn't have the infrastructure to operate Protos (no pilots) and unlike the Jaguars had resources to burn on 'Mechs. The lack of Zelbriggen free units for abusing the Inner Sphere worked itself out when the Kungsarme rejuvenated the tank corps. (Bear tankers are 4/5 post 3075ish.)
elementals and heavy infantry
Protos are closer to small 'Mechs than heavy infantry. For example an Elemental can still move around a building (very strong floors in BattleTech) while a Proto can't. An Elemental can take a guy prisoner, a Proto can't.
War is about more than shooting the biggest guns at each other. That is what we have 'Mechs for. Infantry fills in the gaps where the big guns can't go and are inappropriate.
It is well worth noting there are a lot of situations where infantry is still more appropriate than battle armour. Consider for a moment that Elementals don't have hands making them bouncy Daleks. Where does that leave Protos?