Neither of the three. According to this reply from Bitcoin Core developer achow101, it is not considered "safe enough":
I would say that falls in the 'not want to' category for the reason of the implementation not being safe enough.
Also, more generally, many Bitcoin Core contributors don't consider BIP 39 to be secure. It uses PBKDF2 which is generally regarded to be a fairly weak KDF so it isn't considered to be good for the secure storage of all of your Bitcoin.
Why do they consider it weak from a security standpoint? In other words, how much less secure is it, and are we talking about a worrying threat to a user's coins? A threat that is significant enough to not want to touch it.
Some software (such as Electrum) used BIP 39 in the past but have switched to using their own mnemonic algorithm because of this weakness in BIP 39.
Electrum's seed system came before BIP39. I think they are two years apart. But the older versions of Electrum used something else for seed generation. The seed versioning system we have today came after BIP39 if I am not wrong. At what point did Electrum use BIP39?
As @BlackHatCoiner said they don't consider it safe enough...
You missed the word "don't" in your post above.