Chest and Keys question

So maybe it will become clear when more is revealed later, and maybe it doesn't apply to ranked que, but I wonder why you can't earn the points/parts for this reward if you're solo (like at significantly reduced rates or something, ie 20-30% value of duo que if you solo que)?

Maybe my opinion will change with the new ranked system, but there was something really satisfying about being able to get through promos and rank up solo. I played a second account entirely solo que w/out duo to see if I could get to the same rank as my first account or if I was carried to higher rank by my duo friends. It felt really good to be able to get to around the same mmr and know that I did it on my own. If there's not a personal climb component to solo que and a "yeah I can exert influence on my team at a silver/gold/plat/whatever level" satisfaction of getting the rank it will lose something for me, so if you can earn these rewards in ranked and can't get them even at a super reduced rate when you're solo it kind of hurts. Either I choose to lose that feeling of personal accomplishment that is rewarding and enjoyable, or I lose out on potentially really cool rewards. I hope we get some reasoning on why this is restricted to a party reward, and not just what the rules and requirements are to earn the reward.