Heck... for all we know they do intend to eventually release 5E under the OGL... but just not do it until they've released everything they want to release for the game first-- at the pace they themselves want to release it at.
Cause let's be honest... as soon as 5E material gets opened up to the OGL, we are going to see a deluge of product for it... all of varying quality... all from different manner of "companies" trying to make a couple sheckles off the 5E teat (how's that for a mixed metaphor!). Once that happens... players are going to buy as much crap as they themselves want to spend on it, eventually burning themselves out in the process. The more stuff available via an OGL, the faster that occurs, and the sooner 5E's lifespan comes to a close.
But... if WotC holds off until several years down the road, once they've got a solid baseline of product out on the market themselves already... they might be more willing to let other people enter their playground at that point. The upside of that is they don't flood the 5E market too early, burn people out, plus get their own ideas for potential future products usurped by smaller companies who produce and release them earlier. The downside of that is many people who potentially would want to make product and money off of 5E might not be inclined to try and do so once the game is four or five years old. But then again... WotC might actually see that as an upside, as it weeds out the folks hoping to just make a quick buck off of product-starved players rather than actually producing quality product to help sustain the long-term future of the game. If you start in to produce product for a five year old game, it means both that you think you have a viable product idea that people will buy even after five years of WotC-released product... and you think the game has long enough legs where your product will still be viable (and thus you'll do everything you can to help sustain the legs of the game as well.)
Not exactly the kind of answer many players (and design companies) would want to hear... but quite possibly could be true.
