I like Marlin lever guns. Why wont they make a nice compact well made .454 Casull/.45 Colt like a certain overseas competitor has. You know the would sell like hotcakes! What is the deal? Is Marlin adverse to big sales? If ignoring this oppurtunity is some stupid ploy to promote their own calibers, well that is just dumb. I don't want a Marlin .308 or .444. I want a Marlin .454. Me and about 5 million other brush area deer hunters. What gives?
If you want my opinion, the answer is two-fold: The cost of developing a beefier frame, and the simple truth that I think you might be over-stating the demand. The other cartridges you named (308 ME and 444) as well as numerous other offerings, such as the 35 Remington and good ol' 30/30, will do everything you could possibly ask of a brush gun, so what void would a .454 Casull, in a lever-action, really fill? It would be more of a good thing, I suppose, but "more" is not indicated, based on the historical success of existing brush cartridges.
It would be much like the 348 Winchester, 356, 358, etc, etc...a few people clamor for them, but fewer still will actually buy them, when they are offered. Plus, IF Marlin made the investment in such a gun, it would be noticeably larger/heavier than existing models, which might keep many would-be buyers from coughing up the money to purchase one. If you really want more than the power of a 444 lever-action, you can always shoot 3" slugs out of semi-auto 12 gauge or get a 35 Whelen pump gun. Lots of existing options.
A T/C Encore can be chambered in .454, with a carbine barrel. That would give you an easy carrying gun with some real authority, at woods ranges and allow you to shoot gentler 45LC rounds through it. Only thing missing would be that much-talked-about-but-rarely-relevant quick follow-up shot.