Is there some particular explanation as to why "Marco Polo" is still missing? It's funny, considering Seasons 1 & 2 are otherwise so relatively intact, that there would be an entire 7-part story that's just totally gone.
There are at least explanations behind the other 4 missing episodes from the 1st 2 seasons. "The Reign of Terror" parts 4 & 5 were destroyed by Turkish bombing during the Cyprus civil war of the 1970s. (Parts 1, 2, 3, & 6 only survived because they were being kept in a different building at the time.) "The Crusade" parts 2 & 4 are missing because most of the surviving episodes from that era were recovered from Saudi Arabia. Saudi Arabia never bought "The Crusade" because an Arab TV audience would be unlikely to enjoy a story about the Crusades told from a British perspective.
But what's "Marco Polo"'s excuse?
Honestly, I think a good chunk of its cache & mystique comes from the fact that it's the earliest missing episodes. When you're on a methodical binge-watch of the complete series from beginning to end, this is the first roadblock.
Personally, of all the missing episodes, the ones I'd like to have back the most at this point are "The Highlanders." But I'll settle for animated reconstructions of those instead.