Varför det är en björn, är sannolikt bara författarens preferens.
Den exakta orsaken till livstidsdräkten avslöjas i ett senare kapitel:
There is a scene showing Lala, Saito Hiroki, and Ochiai, with Lala in a life supporting chamber. They talked about Gauna's placenta invading her brain, presumably by injuries from a battle; though there's no adverse effects, she will die without life support, and they weren't able to cure her with their available tech, so Ochiai made her a suit. From the appearance of Ochiai, this episode likely happened at least 7 centuries ago. And, the location of her complication probably prevented an easy body swap.
Varför måste det vara en björn?
Again there's no canon explanation for that. But probably from the need to fit the whole or part of Lala's body inside and the relatively humanoid shape and size of a bear. In the above scene, it also shows a similar suit with a transparent dome shaped helmet, you can see Lala wearing it, that almost resembles the bear but mechanical looking, with the iconic mechanical rotating hand.