Why do cyclocross/touring bikes seem to have longer top tubes than road bikes?

Modern road bikes have moved to compact frames, which was pioneered by Giant. This was do to several reasons. They are stiffer, when all things are equal. They are also a bit cheaper to produce. They are also easier to fit a larger number of people to fewer frames.

Cyclocross bikes do come in compact and more traditional forms. Surly is about as traditional they come. Touring and Cyclocross bikes share the need for stability during dynamic weight transfer. That's why they are longer, as a rule. A longer wheelbase is more stable/less twitchy.

When it comes to you finding your dream bike, I have some simple advice from the dawn of bike fitting. Stand over the bike, and make sure your um...unmentionables have about an inch (for a touring bike and 2 or so on a Cross) before they make contact with that top tube. You can't change top tube height. Stems, seats, handlebars can be swapped out at will. I would also consider the length from the center of the BB to the top of the seat on your favorite bike. Make sure you can replicate that as well.

I hope that helps, and good luck!

On touring and Cyclocross bikes usually the handlebar is higher than a road bike. When the handlebar is higher the reach will be longer (like when you lift your hands they are getting further away from you body). In order to compensate on that longer distance, you can either have a longer stem or, in that case, a longer top tube.

I would have to disagree that long top tubes are more "traditional". Most of my steel bikes, ranging from 1954 to 1986 or so have square dimensions, 57 top + 57 seat. The only one that doesn't is a comparatively new soma doublecross from around 2004 or so, it has a very long top tube. I also find it uncomfortable on long trips due to being too far forward. I have found the same thing with newer steel/touring bikes. I can't explain it except that I think a lot of frame makers for the past 20 years have sort of worshipped rivendell/grant pedersen which started this long top tube fad. I think it's because they evolved from the 90's mountain biking world which used long top tubes. I think it's a bit ridiculous and would prefer a shorter top tube and longer stem. All that said, once sizes get very small, like below 52, there simply isn't enough room for 700c wheels anymore and compromises must be made, one of those is to keep the top tube long. At this point people should really be using 650b wheels instead. Good like finding what you need, and don't compromise on fit!