Retailers were making sure that they didn't over order Wii Us like they did last Christmas IMO. We can see from the numbers that it sold respectably, but I'm sure Nintendo would have been able to get more systems to retailers had they actually wanted them.
Didn't the Wii U sold really well last Christmas? The drought started January 2013 iirc.
As of Dec 31 Nintendo had shipped 3.09m Wii U and according to this site they didnt sell that mamy until late May.
As of Mar 31 Nintendo had shipped 3.45m Wii U and according to this site they didnt sell that many until late August.
As of June 30 Nintendo had shipped 3.61m Wii U and according to this site they didnt sell that many until early October.
As of Sept 30 Nintendo had shipped 3.91m Wii U and according to this site it didnt pass sell that many until mid November.
Basically it took 6 months to sell the launch shipments and another 6 months to sell an additional 820k so based off that its understandable why rwtailers didnt want to order a ton of units. The good thing is that Wii U selling out is showing retailers that there is demand for it currently and if the demand can continue into next year then retailers will be more willing to recieve bigger shipments.