There are no doubt numerous reasons & possible responses to this question, r0b, but I suspect one the main reasons is simply that we are creatures of habit, and we condition our palates.
I daresay that most of us were introduced to whisky by being offered one particular type or style, for instance, a friend offers us a single malt. We then subconsciously make the presumption, "So, this is what whisky tastes like!" Having now tried one single malt, we are tempted or encouraged to try other single malts. After a while, or many different single malts (whichever comes first!), we have conditioned our tastebuds and our brain into asserting that the style/flavours of single malts are what whisky is all about, and it's what we like and enjoy.
And then we are suddenly tempted or offered to try a different style of whisky, i.e. bourbon or irish. Not surprisingly, it tastes completely foreign and strange! And this is where the "scotcher" has to make a choice: He either decides, "Well this tastes too strange or different; it's not for me", OR he decides that this new flavour and new world is worth exploring, and he then trains & conditions his palate to accept other whisky styles.
Being creatures of habit, and preferring to live within our comfort zones, I suggest most decide that it's easier to stick to scotch.
Besides, it tastes so much better !!!!