Why can I no longer find baking potatoes with the thicker skin? 

Many things have changed in agriculture over the years. However, potato skin thickness remains the same, so far as what attribute each potato variety offers. What can be different, and I suspect is the case, is that when potatoes are ‘new’ – that is to say, freshly harvested in late summer and early fall, these tend to have skins that are indeed thinner as the potatoes have not had a chance to completely transition into their dormant stage. At this point (as is much of the Idaho crop in every harvested variety), the bulk of the crop goes into long-term spring and winter storage. At this point the potatoes will develop an even, thicker skin layer which serves to remain in storage for up to nine months or so. Keep in mind that potatoes are a living organism and as such, have certain nuances and characteristics that help them store well and ultimately propagate into next year’s crop.