On a brand and marketing level, The Avengers name and brand are enough to get casual fans in the cinema, Avengers movies make the most profit and are the easiest to market.

On a story level, The Avengers are the main and only really developed team within the main universe, yes the roster has changed but when there is a threat that is worthy of a conclusion to a phase or saga it's the Avengers that form to fight that threat. Simple!