The Mark 4 series are obsolete and absurdly overpriced at civilian retail (best price you can find without mil/LE discount). The Mark 6 series are pretty good scopes and not outrageously priced for their competition, but there are too many reports of issues with them, and there is a lot of really good competition in their price range. The one I owned also felt very lightly built compared to any of its competitors. The Mark 8 series are probably great scopes, but at a price level where there is competition from the truly world-class brands (S&B, USO, Premier/TT, etc.) and very few people have the cash to buy any of them.
I say this as someone who has owned examples from the Mark 6, Mark 4, VX-6, VX-R, and VX-2 lines, and has only seen fit to keep a VX-R. The Mark 6 would have been completely satisfactory except for nagging worries about its durability, which aren't acceptable for a nearly $2k scope. (Example - the lock ring for ocular focus - if I remember correctly is less than 1mm thick and probably weighs one gram. Doesn't inspire confidence. Scope is a paperweight without a focused reticle.)
However I would also agree with Arc Light's statement that preferences here can be a bit of a herd mentality, what's fashionable one year vs. the next, or however you want to put it. But I just don't think anything Leupold below the Mark 6 line is competitive right now, and there is a lot of extremely good competition once you hit that price level.