When you don't have the cash to do it yourself.
In case you haven't noticed (and it's kind of hard not to), Mitsubishi is desperately in need of new models for its US lineup. Problem is the development money isn't there for Mitsubishi to go at it alone, hence the need for a partner. Specifically, Mitsubishi needs new sedans, both compact and midsize. The Lancer is still on sale but it dates back to 2007. The Galant was retired in 2010. Previously Mitsubishi was in talks with Renault regarding a partnership arrangement but those have reportedly fallen through.
Instead, Mitsubishi has turned to Renault's alliance partner, Nissan, for help. Nothing is set in stone yet as discussions are still ongoing, but Mitsubishi is reportedly quite desperate for a Lancer replacement. That makes complete sense because, despite its age, Lancer sales in the US dropped by 15 percent to just 16,495 units last year. There's actually been a Lancer sales increase so far this year compared to this time in 2014, but that's obviously not good enough. And no, a new Lancer, even with Nissan's possible help, won't be given the Evo treatment. Bummer.