Most pedal compressors are terrible, that's the problem.
This. The commonly used compressor pedals (MXR, etc.) are crap if you ask me. And the result for many guitarists who try these awful compressor pedals is that they don't understand how good compression can take a good tone and make it a great one, especially for cleans, because the crap compressors they've tried are robbing them of their precious dynamics. And they're right.
One rule of thumb for compressor pedals is if it's got less than 3 knobs, definitely avoid it. For me, the key to getting a good compressed tone is delaying the attack, so the front end of your notes stays unaffected (leaving your picking dynamics alone for the most part) while still giving you the creamy saturation that you want without adding gain. But just because a comp pedal has 3+ knobs doesn't mean it's going to be good. Even with an attack rate control, many pedal compressors still suck ass, because they don't attack smoothly enough. Like you hear a plunging sound when the compressor kicks in. That's not what you want, either.
Again I have to give Rocktron some love here. Their pedals are all pro rack gear pedigree, and their compressor pedals are fabulous, at least with the attack turned up high (to make it kick in more slowly). No weird plunging as the compressor kicks in, and it affects short notes and picking dynamics little if any, while still giving all the deliciousness to the notes and chords you do want to sustain. Plus they're fairly cheap.