I found a paper which measured the energy of the transition [0], but it doesn't talk about why it's so low. Might be a starting point if you have more time to read than i do, though!
EDIT Hmm [1]:
> Interestingly, the existence of a nuclear excited state of such low energy seems to be a coincidence and there is currently no conclusive theoretical calculation that allows to predict nuclear levels to this precision.
And there is a paper with a ton of detail and some nice diagrams of energy levels [2], but i'm not sure it really gets at "why".
EDIT Hmm [1]:
> Interestingly, the existence of a nuclear excited state of such low energy seems to be a coincidence and there is currently no conclusive theoretical calculation that allows to predict nuclear levels to this precision.
And there is a paper with a ton of detail and some nice diagrams of energy levels [2], but i'm not sure it really gets at "why".
[0] https://arxiv.org/abs/1905.06308
[1] https://link.springer.com/article/10.1140/epja/s10050-020-00...
[2] https://iopscience.iop.org/article/10.1088/1361-6455/ab29b8