Well, if we're going to switch to IC arguments, I'll point out how large the anti-vehicle bias is in most Clans, including the Wolves, especially pre-Invasion. There isn't a lot of honor to be had disabling tanks, so it's especially odd that they'd build an assault mech, even as poor an assault mech as the Man O' War, for the role.
I don't think they did. I don't think the Man O' War was designed for that purpose. It has 16 DHS inside the engine, after all. If I had to come up with an in character rationale for it, it would be something like this.
--The Man O' War was designed to fulfill the same kind of role as the MadCat. Given the complexities of "real" mech design, apparently the people in-universe haven't figured out the common break points that every player knows (i.e., 80 ton mechs are inefficient, but nobody knows that). It was a very solid design with good speed, good pod space, great heat dissipation, and decent armor. It forms the core of the Wolf Clan's "fast cavalry" units.
--Then the MadCat is created, and it's just better. The Man O' War starts becoming a less preferred design, but the Wolves still have a lot of them and so you gotta use 'em.
--The Clans invade the Inner Sphere and find that their main combat forces keep running into tanks and other conventional forces. Most of the Clans gave these up long ago, but the FedCom especially seems to love them. So now the Clans need something to get rid of these pesky annoyances that their heavy cavalry forces keep running into.
--Some commander orders his Man O' Wars to carry an anti-vehicle configuration when they're facing FedCom RCTs. It is very effective and it quickly catches on. It ends up becoming such a common move that the FedCom identifies this as the "primary" configuration for the mech. It's the one they see the most. Once most of the vehicles have been dealt with, mechwarriors switch to the "A" configuration, or one of the others.
That's going to be my head-canon for it, anyway.