Is there a systematic reason to why some games can be gift emailed and others cannot?

Hi guys,
I’m finding it pretty frustrating that I can’t seem to send any game via email but only a select few. There seems to be a complete scattergun approach about which method of delivery is available for what games.

Some have email only, some have bnet/realID friend only (with cooldown to boot).

Am I missing a way to send any game via email or is this just classic Blizzard consumer-oriented wisdom saving me my own desires once again?