I'm sure Nintendo aren't keeping the specs a secret from developers.
And TBH I reckon specs are about the last thing developers will care about, unless you can boast about them and prove you have something of worth to attract the hardcore gamers. A dev I was speaking with the other day was telling me no-one they knew was developing for Vita as they don't see a market there, despite its awesome specs. Nintendo needs to convince devs that they can develop for Wuu economically and will have a large, game-buying install base to buy games. I don't see that that'll be facilitated by telling the world how uninspired their hardware is.
The Wii has a much larger user base than X360 or PS3.
How many of them are developing for the Wii?
Why did they stop developing for the Wii, or why didn't they ever develop for the Wii?
Probably, it's because they couldn't make good-looking games for the Wii, because the Wii's hardware is subpar.
Because good-looking games sell, no matter how people like to underestimate specs. The Wii made Nintendo lots of money until 2009. Until then, it's a downward spiral of losses and absent mindshare.
And that's also a possible reason for them to not ever support the Wii U. Because it won't be able to do graphics as pretty/realistic as PS4/Durango.
Nintendo's "1,5xLast Gen" technique worked great because they were tapping on to a new market: the casuals.
But all those have their Wiis gathering dust for years and have moved on to ipads, iphones, androids, facebook games, kinect/move, etc. That market isn't untapped for them anymore.
Besides, do you have any doubts that if Nintendo had shown some "4x faster than Xenos" GPU market-specs along with a Samaritan-like real-time tech demo, even with the exact same game lineup, that their shares wouldn't have risen instead of falling?
The fact that they won't say what's inside the console is making everyone believe it's not more powerfull than the other two consoles and that's costing them lots and lots of money on credit.