NO reason? What you mean is that there is no reason you agree with that the screen is the way it is, but there are plenty of practical reasons that the screen is the way it is.
Don't get me wrong, I can't wait to see a mini with an improved screen, but I think Apple did the exact perfect thing with the mini by optimising and maximising its one big and defining feature: form factor. The mini is now defined by thin, lightweight with great battery life. Retina (or any improved display) will always be constrained by those product attributes, otherwise the product will lose focus and not be a "mini" (as we know it) any longer. What this means is that when they do improve the display, they'll do so within the parameters of thin, lightweight and great battery life - what a great device that will be!
Personally, I'd rather it start from here and go there, rather than start from or end up at a position of fatter, heavier and thicker either today or tomorrow. The biggest most wonderful feature of this product is its form factor - Apple did it right, though some may not be happy with the decision they made, millions do agree it was the right decision (not the least of which are iOS developers, who changed nothing in their apps yet were able to sell them to the millions of new mini owners on day 1).