I doubt they felt that there was a big enough market to cover the cost in doing so at the same time as the regular release. Especially as you can't just grey-scale the head sculpt, you'd have to redo some of the clothes, the hands, and the accessories as well. Plus they probably didn't want to split the market, either.
As much as I personally like the idea of it, it seems like a lot of work to risk at this point on what would likely be, at best, very little profit for Big Chief. Plus, if they did do a B&W Hartnell, and then even if it didn't sell to minimum expectations, all those that purchased one would
still [strike]expect[/strike] demand them to make a matching Troughton anyway, or else there invariably would be never ending complaining on the subject. Right now they likely thought it best to avoid such potential headaches, and just go with the regular. After all, they've shown that they will do key variants if enough of a demand for them is shown to be there, so who knows what the future might bring...
With that in mind, it does make for an obvious variant if they ever do have the need/demand to revisit Hartnell for another release one day. Particularly after how well received the release has been, so far. Perhaps the smarter move, if they ever do plan on doing so, is to just wait a bit and do a B&W variant release as a set with a black and white Patrick Troughton included as well (after his regular colour version has also been done, of course), that way, those that purchased the colour version may end up buying both (which is good business for Big Chief), and you also get both B&W variants knocked over at once, for those that are interested, so there's no risk in not being able to complete the early era black and white look due to things such as lower than expected sales on one or the other.
Regardless, right now I'm just happy to have any Hartnell on my shelf at all, let alone one this good.
Seems its happened to quite a few collectors. Mine had come out of the tray in transit and had broke 
(jk)
