The current title, Main Page, in the main namespace, was created in the default namespace before pages were divided into "Wikipedia", "Portal" and so forth, and has continued there mainly due to historical inertia. Several previous proposals to move the page failed due to rejection or lack of consensus.  See previous discussions in archives 67 (April 2006), 87 (January 2007), 89 (January 2007),  90 (February 2007), 114 (December 2007), 115 (December 2007), 123 (May 2008), 125 (July 2008), 128 (October 2008),  129 (October 2008) and 143 (August 2009). 

Several other Wikimedia projects also use "Main Page" as the default home page, like Meta-Wiki, Commons, Wikibooks, Wikinews, and Wikiquote. Some however do not: the default home page for Wiktionary is "Wiktionary:Main Page" and the one for MediaWiki is "MediaWiki".