I know for me, I can only work on what's special and interesting to... well me! That's why I do it. I'm a graphic designer for a living, but making/selling shirts is my side biz and I wouldn't be doing it if I didn't enjoy it.
Give an example. I was born in 1971, so you can probably deduce what my inspirations are as far as pop culture from my past. I designed a "Shogun Warriors" shirt based on the 2 foot tall toy robots made by Mattel I had as a child. They started selling like hotcakes and business was good. I assume mostly because it was an untapped area (at least that specifically) and I'd like to think partially because I had a good design.
My friend says to me, "Hey, you know what you should make next? Robotech shirts!"
I don't care about Robotech or any aspect of it. Therefore, I wouldn't want to spend hours conceptualizing and drawing an idea, execute it, start selling and then have to look at them when I produced them!
To me, there's no fun in that. Drawing the Shogun Warriors was fun because it brought me back to my childhood and I could really get into the design and have fun with it.
I like to share what I like with others who will appreciate it. I guess I'm selfish like that. If it was just about money, I'd be doing something else.