It's a good question and the one that has no simple answer... or any answer.
Don't forget that "Mark's guitar playing" is not something he invented single handedly, it has elements of J.J. Cale, Chet Atkins, B.B. King etc... From my understanding, the term "it sounds like Dire Straits" originated long after Dire Straits debut album and everybody was like: it sounds like J.J. Cale, Chet Atkins, B.B. King etc... He got extremely lucky since he was born in the right time and in the right place, which obviously nobody can imitate. When I was a kid I wanted to play on every guitar with my fingers long before I knew who Mark Knopfler is, I just dig the sound like he digs it, so it's not so much of a unique idea to say the least.
Nobody can imitate someone's guitar playing or songwriting, but if you're talking about "sounding like MK", it's hard because it rely more on skill rather than on gear (same thing with many many many of great players, by the way). But you can start to "sound like MK" by taking a Stratocaster guitar, any Strat out of the box and engaging bridge and middle pickups on any amp. It's like if you want to sound like Django you buy a Selmer-Maccaferri style guitar and boom — you sound exactly like Django!
I don't think that it's really hard to imitate Mark's guitar playing as much as it has no purpose, because he already sounds like MK enough. It makes sense if only you're going to become so great that like with Mark himself, everybody will forget you imitating and remember you becoming yourself, but it's hard to do, because MK is a genius and that's really what is hard to imitate.
