Nike don't own Umbro.
And now i'll try answer in such a way that you may understand. The main reason will be numbers, and specifically how big the profit margin is for Nike; they will take into account previous sales figure, estimated sales prediction, sourcing, producing, shipping, shelf expectency etc etc of the product. They'll have a profitable figure for each shirt (could be £5, could be £10); now after calculating all the previous items listed an average cost per shirt will be generated, if this match there proftable figure then it won't be produced. Basically, there is no market for the item.
Now, if you don't understand this, your next question will be 'blah blah blah but Umbro and LCS did and they must have made money'. And yes, you are correct, but maybe, just maybe there margins are lot lower than Nikes and any profit is profit to these companies; after all Nike are global leaders in this field.
Compare it to Tesco and 99p stores; Tesco have a reasonable profit on most goods; when negotiating with suppliers there will be some leeway on prices (probably £s), they know what sells, what people purchase (based on Post Code and avg salary) and will stocks stores accordingly. Then we have the 99p stores, every item is negotiated down to the penny, thats were the profit lies; make a penny on an item and its profit, save a penny when negotiating its a profit.
And thats the difference; if there is no call for the product - Nike won't manufature it unless its really cost effective to them; on the other hand, the 2 mentioned brands wouldn't care less because it could be a small profit for them.
As do all manufacturing companies; it won't be worth there while. But that being the only reason; i doubt it very much