using mac book pro why can't send i msg to non i phone users?

iMessages can only be sent to Apple devices. An iPhone can send both iMessages and SMS/MMS messages because it has the necessary hardware and a text messaging plan from a cellular carrier which a Mac does not.

If you have both a Mac and an iPhone you can use Continuity and it will use the iPhone to send an SMS/MMS message from the Mac via the iPhone: http://www.apple.com/macos/continuity/

You can use an app like Skype orn even Face Book to send messages but the recipient also has to have the same app

The Apple Messages app on a Mac could never "text" to a non-Apple device unless Continuity was set up and that requires the using the Same Apple ID on moth an iPhone and on the Mac