As others have discussed, I believe it comes down to the complexity of the amps, the current market value of the real Marks and the demand for them.
First, outside of Bugera's attempt at capturing the essence that is the Mark, it will more than likely not be cost effective for manufacturers to create Mark clones that are comparable. That leads me to my next point.
Without being able to keep prices low, how could a manufacturer of clones compete with the used market? It is likely that a consumer will consider this: "If I can buy a used Mesa Boogie Mark IV for $1200, why would I pay $1400/$1500/$2000 for a clone?" Buyers are likely going to prefer an item from a reputable manufacturer than a newcomer.
Finally, how many people really desire Mark series amplifiers? Go into a Guitar Center and ask anyone what they think of when they hear Mesa Boogie. Chances are you will repeatedly hear the word Rectifier. The Mark series is relatively unknown and unpopular. Outside of Dream Theater and '80s fan boys, the Mark amps are not all that well known. Unfortunate as it may be, this is the case.
Finally, how many accurate and affordable Bogner clones are on the market? What Bogner has done with the Marshall platform, Mesa Boogie did with Fender. There are plenty of Marshall and Fender clones out there; however, there are very few clones of amps that took these basics a step further.
Oh, and Randall Smith and Mesa Boogie love patents.
