Why did C.S. Lewis use Turkish Delight as temptation for Edmund?

Beside the delightfullness of some brands of Turkish Delight according to some of those who have answered here, there is also the Turkishness of Turkish Delight to considered.

If Communist Cookies, or Nazi Nougat, or Stalinist Sweets, or Enemy Eclairs, or Anarchist Apple Fritters, had been a popular band of candy or sweets when Lewis was writing, he might have chosen them for Jadis to tempt Edmund with, for the sake of their sinister sounding names.

But if Lewis was limited to using a real and familiar type of sweets, I guess that the name of Turkish Delights might have had the most sinister and hostile connotations of any real type. Various Turkish realms had been feared threats to various Christian and/or European societies for six hundred years up to the Great Turkish War of 1683 to 1699 when their conquests in Europe began to be reversed.

While I support the answer of Mwr247 for important context (even if it largely lifts from another source that lacks a clear CC license), I came across an interesting answer for this question years ago at a diverse table of people from many countries.

The Turkish representative had brought a box of Turkish delight, imported from Turkey, and thus labeled in its original Turkish language ("Lokum"). This was touted as the real thing; I'd finally get to learn after years of waiting just what Edmund (Lewis) was referring to.

The Danish representative, arriving a few moments later, laughed and commented that in his language, the word means "toilet."

So even linguistically, we were set up for the experience of the dessert being a welcome treat but not nearly as amazing as Edmund described it, in comparison and context with a modern peacetime of relative abundance where a wide variety of sweets and rich foods are relatively easy to come by.

If Lewis knew this, this particular choice of treat could have been a hidden reference to something which seems quite attractive on its surface but has a much dirtier true meaning underneath. With evacuated Danish children in wartime England, real Turkish delight something that would have to be imported (according to Leah Schnelbach's article quoted by Mwr247), and the higher average number of languages Europeans are familiar with (compared to residents of other English-dominant countries), it would not necessarily be that unusual for Lewis or at least some of his readers to pick up on this connection.

His writing style puts meaning on so many different levels and layers, that I would not be entirely surprised if this was intentional. Also, he could not have conveyed that meaning with any other choice of treat, so it lends a bit more weight to the possibility that the extra layer of meaning may have been intended.

In a later reading of the particular section quoted in the question, I also interpreted "Turkish Delight" as a code for addictive drugs (e.g. opiods) that also make one forget one's manners, think only of trying to shovel down as much as one can, where the more one consumes the more one wants to consume. Lewis's goal of embedding morals and lessons important for young people to learn would likely have included strong cautions against this sort of substance, even if is something that sounds exotic and tempting, because such drugs lead to one's downfall and hurt one's family.

Possibly due to how the writing works as Turkish delight is a very specific food for a westerner to imagine.

If I say chocolate you are imagining a vague concept of twenty plus types but you have probably only had Turkish delight a few times and it makes the reader imagine the food with much more clarity.

It should be remembered that until Edmund asked for Turkish delight, Jadis had very likely never heard of it at all, and so the enchantments she used to create it were running on his ideas about it.

It was already Edmund's favorite candy (or he would have asked for whatever his favorite candy was and we would be talking about that), and he could have been envisioning it as more enjoyable than his actual experience warranted.

And since the whole point of offering it to him was to hook him into her schemes, she amped up the pleasure of eating it.

The only reason that he could not stop eating it is because it was enchanted.

As I recall (having read the book recently, though I don't have it at hand now) there's a point where Edmund is looking longingly at the empty box, and the witch is not surprised, since the treats were enchanted.

The reason Edmund forgets his manners (and everything but stuffing as much candy in his mouth as fast as humanly possible) is not the fact that Turkish Delight is the greatest confection in existence, but the White Witch's magic:

At last the Turkish Delight was all finished and Edmund was looking very hard at the empty box and wishing she would ask him whether he would like some more. Probably the Queen knew quite well what he was thinking; for she knew, though Edmund did not, that this was enchanted Turkish Delight and that anyone who had once tasted it would want more and more of it, and would even, if they were allowed, go on eating it till they killed themselves.

This is apparently something she's done before, probably with other foods, as Beaver specifically calls out her food in connection with treachery.

"He had the look of one who has been with the Witch and eaten her food. You can always tell them if you've lived long enough in Narnia; something about their eyes."

So while you can look to the other answers for the reason Lewis picked Turkish Delight over say, toffee, keep in mind that this was never supposed to be a realistic impression of how good Turkish Delight tastes.