Thomas Voegtlin(creator of Electrum) removed himself from the authors after multiple disagreements with the BIP39 design.
If you are not aware there was quite a bit of drama on the Bitcoin mailing list around '13 when the standard was being proposed.
For context, Electrum was the 1st deterministic Bitcoin wallet that allowed for a BIP32 seed to be backed up into 12 words, and then came BIP39.
In all fairness BIP39 was kinda rushed, and even though valid criticism was provided regarding it's shortcomings, they were ignored.
Electrum has its own backup scheme that supports versioning & is not dependent on a dictionary, vastly superior IMO.