I've seen lowercase wording used by people who don't want to put effort into something. Personally, I consider it a sign of dismissal towards the audience. If you see this coming from someone, it might be an indication they don't case. This is based purely on personal experience.
Maybe that's why it's used ? Twitter is just addressed to the public, so it's not official in any way, it's just communication. The lowercase casual style is only used to drive empathy probably.