It's not that real-world Turkish delight is bad, per se, so much as that Lewis makes it sound so life-changingly incredible that the real-world version is bound to disappoint by comparison.
On the one hand, it was mind-numbingly delicious because of the White Witch's magic making it incredibly addictive, rather than simply the virtue of being Turkish delight.
But why the choice of Turkish delight? A combination of status-signalling and unavailability [1] (post-WW2 sugar-rationing was still in effect when the book was written).
