I could see Activision not knowing how many guitars to produce and greatly underestimating demand...
However, did anyone think Nintendo would be able to sell less than 4 mil within the first month if they made that many?
That said, that's probably the main reason why they delayed the launch by almost half a year... Not even 5 months of extra production couldn't make a dent. Hmmm... the scary thing is... that means at this rate it will be sold out for at least a couple of years....