It's helpful to consider in context of the time. Rationings were in effect in WWII England, and Turkish Delights were a particular delicacy that was popular at the time and hard to come by. Also consider that food in general was less sugar packed than many of the options we have today, so many foods at the time that we might consider "disappointing" would have had a much different impression of the people of their time. From an article a couple years ago regarding this exact question:
The question of Turkish Delight often becomes still more perplexing when a young Narnia fan actually eats the stuff, and finds that it does not live up to Edmund’s rapturous praises. As with so many things in pop culture, the answer lies in the context, and since we’re living in a beautiful future, an academic article has stepped in to tell us all about the importance of Delight.
According to food critic Cara Strickland, the Turkish sweet cast an intoxicating spell over late-Victorian England. Made from a confection of rose oil and sugar, the candy is simple on paper, but proves extremely difficult to make – no matter how Western Europeans tried, they never quite replicated it. Thus, if you wanted real Delight, you had to import it from Turkey, which got expensive fast, so that it became a marker of either status or indulgence in much the way the way coffee had a century earlier.
Of course just as costs had gone down, the outbreak of World War II and its subsequent rationing meant that the candy was harder than ever to come by. Perhaps this is why it became so significant to Lewis? As he welcomed refugee children into his Oxford neighborhood, he thought back on the candies and holidays that had marked his own childhood.
It makes sense that Turkish delight would have been on Lewis’s brain as he crafted a book where Christmas features as a main theme. In Narnia, it is “always winter and never Christmas,” a product of the White Witch’s evil magic. It makes sense to draw a parallel between this dismal fantasy and the stark realities of wartime. Rationing extended to timber, which made Christmas trees harder to come by, and confectionery rationing didn’t end until February of 1953—still well before the end of sugar rationing later that year. When the White Witch asks Edmund what he’d like best to eat, it’s entirely possible that Lewis was answering for him: the candy that would be most difficult and expensive to obtain. Edmund isn’t just asking the witch for candy, he’s essentially asking her for Christmas, too.
Why Was Turkish Delight the Ultimate Temptation in C.S. Lewis’ Narnia? - Leah Schnelbach
The article I've referenced summarizes and cites from an article written by Cara Strickland. She goes far more in depth into the matter, including pointing out Lewis' fascination with Turkish culture (Aslan is apparently the Turkish word for Lion, for example). It's far too long and detailed to include though, so I recommend checking it out directly if you want to go deeper.
So it was essentially both a cultural delicacy at the time, and as the article speculates, likely something of specific connection to Lewis himself.
