There is already a GitHub issue open which describes the problem pretty well.
Thunderbird Mobile does not enable encryption by default, when the public key of the recipient is available.
Example: I've setup Thunderbird Mobile and OpenKeychain for my mother. Now she can send me encrypted emails. But I told her, she has to remember to click on the lock icon before she sends the email. She already forget that at least once.
As we all know, defaults are important, because most people don't change them.
I think encryption for recipients with a public key should be on by default.
Thunderbird Desktop already does this the correct way.