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My actual reasoning for infantry: Almost every BTech story is about BattleMechs, for obvious reasons. I wanted to highlight something a bit different!
In Universe Reasoning:
Riley is an agricultural engineer from a family of agriculturalists who raise goats, llamas and sheep for wool production, to turn into textiles and finished soft goods. While lucrative, and the family is historically wealthy, that wealth was in the form of JumpShips and DropShips to transport their goods, basically a merchant family. No BattleMechs, no history of BattleMechs. That said, facilities need guards and it's cheaper to have your own. Warehouses and DropShips need people to guard them, and while BattleMechs are a deterrent, most space ports will already have them. But guards with rifles can patrol in the building without turning the building and it's contents into rubble. Guards with APCs can escort transports on agricultural worlds with no SLDF. During the Reunification war, their JumpShips were "purchased" by the SLDF for additional lift capability, and while the windfall (taxed of course) was huge, losing their transport capability was a major blow. This was due to the family being seen as "disloyal" for arguing against Reunification and against Star League propaganda about the Periphery. As a result, they were now even more dependent on their own security to protect what they had, but also didn't have any funds for any heavy machinery.
There is wealth and then there is WEALTH.
A big part of what I'm hoping to explore through Riley is the costs of owning and operating a military force. She has some super tough decisions to make as she balances her desire for vengeance against the realities of logistics.