why we are not using one routing protocol like OSPF for example for all autonumus systems.
To be blunt, in my network, I will run whatever I want; nobody outside my network has any say in it. When my network connects to someone else's network, we have to come to an agreement. For decades now, The Internet™ has agreed on BGP as the inter-AS routing protocol.
(You might find a few odd locales using IS-IS, but that's still a local agreement.)
I could give you a very thorough long-winded explanation that you may/may-not understand... Instead I will say the following:
- Scalability: OSPF, EIGRP, and other routing protocols do not scale to levels needed efficiently, only BGP does this.
- Control: No routing protocol offers more means of manipulation than BGP.
- Security: Protocols like OSPF running globally across the internet would introduce significant security risks.
That's just a few reasons.