Web API VS WCF SOAP

Consume data on HTTP from clients like mobile, JavaScript, Windows applications, etc.
WebAPI is the technology by which you can expose data over HTTP following REST principles.

	WCF SOAP	WEB API
Size	Heavyweight because of complicated WSDL structure.	Light weight, only the necessary information is transferred.
Protocol	Independent of protocols.	Only for HTTP protocol
Formats	To parse SOAP message, client needs to understand WSDL format.	Output of WebAPI is simple string messages, JSON, simple XML format, etc. So writing parsing logic for that is very easy.
Principles	SOAP follows WS-* specification.	WebAPI follows REST principles. (Please refer to REST in WCF chapter.)

WCF and WebAPI also can implement REST, why WebAPI

WCF was brought into implement SOA, the intention was never to implement REST. WebAPI is built from scratch and the only goal is to create HTTP services using REST. Due to the one point focus for creating REST service, WebAPI is more preferred.