Why are we using SOAP over HTTP in Web-Services ?

Your question is a bit short, could you elaborate? SOAP is an important (but maybe a bit complex) standard for implementing webservices, and HTTP is an important standard for communication over the internet.

well, web services is there to enable interoperability across heterogeneous applications. Http is supported by almost all environments and allows communication between systems over network in a simple yet more secured way without need to compromise firewall settings. SOAP is just a proven XML based protocol to communicate between applications using XML messages. Hope you are aware that XML is simple structured text and is language/platform neutral. So SOAP over HTTP is a natural choice for web services. Subramanian, Kumar kumar at eminenttech dot com J2EE Architect Eminent Technology Solutions (ETS) Software / Portals / Alfresco / Outsourcing / Proteomics www.eminenttech.com

Perfect answer, Just to add that HTTP traffic is not been blocked by firewall so when we are using SOAP over http we don't have to do any config....So SOAP messages are easily sent as a payload of HTTP message what is been refered as HTTP Tunneling