See here: WCF Services and ASP.NET

Within an AppDomain, features implemented by the HTTP runtime apply to ASP.NET content but not to WCF. Many HTTP-specific features of the ASP.NET application platform do not apply to WCF Services hosted inside of an AppDomain that contains ASP.NET content. Examples of these features include the following:

HttpContext: Current is always null when accessed from within a WCF service. Use RequestContext instead.