Because it's pretty well established that Tennant loves Zygons (they're spooky, they've not been done to death yet, they're Scottish...), but here's a bit of an old interview in which he's pondering the technicalities of bringing them back. It sounds like he was already sounding out the possibility of a good, not too expensive monster even back when he was on the show, so maybe Moffat picked up on this when he was brainstorming for the special. [Neil Gorton, mentioned below, said in an interview that the two monsters he most wanted to make were Davros and the Zygons, and he's already done Davros.] Full text transcript available here, video below.
Q: What was your favourite ‘Doctor Who’ baddie when you were a boy?
A: I didn’t have a favourite particularly, I liked the fact that each week, each story there was something new, and I think that was my favourite bit, and I wanted it to be more horrific and more grizzly than the one before. So I don’t think I had a particular favourite, I liked the grizzly ones, the really weird-looking ones… the Zygons, the Sontarans. I mean, there were some really brilliant designs, the Zygons are case in point, they managed to make it not look like a human being, they had these huge coned heads and they made the faces small and scrunched up, and they did things. I mean, I was talking to Neil Gorton who does the prosthetics on our show, who designed a lot of our monsters, and he was saying that were we to do the Zygons again there’s not much to do, it’s a perfect bit of design. The technology of these things was a lot more primitive then, just because it was thirty years ago, and you couldn’t really better what they came up with, and I think that’s a testament to the talent of the people who are making the show now, and were making the show then.