Its pretty simple: 1. Instapaper was very popular. 2. Gruber links to him a lot, and he writes well. More cynically, he has modeled his writing style after

Its pretty simple: 1. Instapaper was very popular. 2. Gruber links to him a lot, and he writes well. More cynically, he has modeled his writing style after Gruber's, so if you want more of a Daring fireball fix, you read Marco. I don't think Marco's opinions really hold anywhere near the weight of John's. Some just have an appreciation for this style of writing.