Probably because they weren't well supported in the West at the time and Apple didn't want people sending messages and emails from their iPhone with missing glyphs. It's one of those things you need a consensus for, both the sender and recipient have to support it. It's not something like iMessage where Apple only has to worry about their own ecosystem.
They weren't standard yet. Sending messages containing Unicode Private Use Area glyphs that not even other Apple products could display, let alone other vendors' (non-Japanese) phones, would be an interoperability nightmare.
Probably because they weren't well supported in the West at the time and Apple didn't want people sending messages and emails from their iPhone with missing glyphs. It's one of those things you need a consensus for, both the sender and recipient have to support it. It's not something like iMessage where Apple only has to worry about their own ecosystem.
They weren't standard yet. Sending messages containing Unicode Private Use Area glyphs that not even other Apple products could display, let alone other vendors' (non-Japanese) phones, would be an interoperability nightmare.
They were in Softbank’s private use area, so no interop story really.