my assumption is that sales are low from lack of games, people not caring for the gamepad concept, and people who view it as last gen.
I actually consider the Wii U a hybrid generation (7.5 gen) as its a mix of new and old hardware (and features)
old: higher clocked 3 core variant of the Broadway (Espresso)
hybrid: newer RadeonHD based GPU, has features comparable to the RadeonHD 4xxx series (Latte)| roughly faster than the 360's (Xenos)
old: pro controller: an xbox360 like controller with the same features as a current (7th) gen controller, but uses digital buttons instead of triggers (like the PS1/N64 had; PS2's DualShock 2 had pressure sensitive analog buttons)
old: uses Wiimotes and the IR sensor same as the Wii, no improvements there
new: has a really nice Gamepad that acts as a second display that can act asynchronously to the main one.
new:uses a Blu-ray like disk that can hold up to 25GB (single) or 50GB (dual layer;i see no reason the Wii U would not work with dual layer disks as Nintendo was able to make the Wii work with dual layer DVD's, most of them anyway)
really old: no DVD Playback. even the original Xbox and PS2 could play back dvd's.(the licensing expense excuse may have worked back in the ps2 days, or trying to keep the Wii affordable years ago, but it does not cut it now in 2013, especially for its 349.99/299.99$ price tag)
new: has HDMI and up to 5.1 PCM multichannel audio support(possibly 7.1?)
old: can use external HDD to expand storage (old as the 360 has had this for years)
old: uses the same USB controller from the Wii, just enhanced by having all 4 ports accessible, vs just 2. WiiMode can actually talk to all 4 ports (correct me if i am wrong) when the normal Wii only supposedly only had 2 (it actually has 4, only 2 were utilized)
old: games are limited to 720p, or 1080p for less demanding games (the PS4 and Xbox One will both drive games near or at native 1080p)
FAIL: WPA2-PSK is not supported for WiiMode apps. (big fail here as Nintendo could have enhanced the Wireless drivers to allow WPA2-PSK2 as the Wii U's wireless chipset does support it.
FAIL: gamepad support's wireless N 5Ghz but the Wireless for Internet is still only 2.4. ( why not allow both then worn users that gamepad range may be reduced when using 5ghz for internet access. (my room gets good 5ghz reception but poor signal quality (not strength) on 2.4.)
WIN: only 350$ vs 400$ or 500$
WIN: only consumes ~50 watts while in a game. not even the new 360 E redesign can touch that.
new/WIN: really nice MP4 H264/AVC support. upto profile 4.2 and 7 reference frames. thats better than the PS Vita and PS3.
old/FAIL: No mp3 support: Photo Channel 1.0 for Wii had it. ps3 and 360 do. the ps4 and xbox one will too.(may not seem bad but plenty of H264 videos use mp3 audio streams)
old/FAIL: lack of MPEG4(non H264),AVI, and WAV. (xbox 360 and ps3 could play both, if we had WAV support, we would have a way to listen to lossless music as we could convert FLAC to WAV for playback when needed.)
new: next gen gameplay: the gamepad does offer this, and does it well.
old: 1 profile at a time can be signed in, cannot use 2 profiles for split screen. (even the original xbox could do this for games)
old: 1st party games in stereo (really :facepalm, at least use the other channels for music/ambient sounds) this is not the n64 here.
so, the Wii U is a hybrid generation console (to me at least, and probably others, but the majority of naysayers will still label it as a definitive last gen console)
mixed gen hardware|next gen gameplay = Wii U
NOTE: I love playing my Wii U, and enjoying Amazon Prime, Youtube, and its off-screen compatible ability to play H264/AVC with a decently wide range of profiles for a console)