Yes, it's a really good question.
Possibly the reason is that developers think like me:
The iPad is designed for productivity tools and a smartphone for communication because it's always with you.
Thomas described an unusual situation by moving around between so many devices. All the mentioned services are designed as short
messaging services and not for long intense conversations which should be archived or synced across all devices.
Nevertheless it would make sense because a conversation e.g. via WhatsApp would be more convenient with a bigger screen size.
Thanks for your post.
Quality as usual and designed for a content network
like G+ :)