Tom wanted it, he believes the Monarch is probably Lancer's most iconic frame. There were early sketches where I suggested using the Nelson, a suit I personally like more and felt occupied a similar space to the Monarch in terms of recognizability. In the end we went with the Monarch because we wanted the cover mech to be a "canon" design, and the Monarch's standard red color scheme makes it stand out.
The funny thing is I've drawn the Monarch so many times but it's not even my favorite Lancer design! If I had it my way, I'd be drawing HA and IPS-N more often. You'd see nothing but Tokugawas jousting with Nelsons. I'd be drawing a Sherman fistfighting a Tortuga in an ideal world.