The LetsEncrypt staff shared some insight into this with the Community Moderators earlier this year. It is still on their radar and something they want to offer, but it is not likely to happen in the near future. They have a backlog of higher priority items to get through, and will then have to address some staffing implications before offering this.
It seems that ISRG is basically at an operational sweet spot of 90 day lifetimes right now. As shorter lifetimes will mean more certificates, higher server load, and would make any outage even more critical, they would need to staff up engineering resources to handle everything with the same level of attention and reliability. They don't want to overextend their personnel.