It's an observational bias, based on how they find these planets. If Neptune were as close to the sun as Venus or Mercury it would have more pull on the sun and create the "wobble" that scientists use to detect the majority of exo-planets they've found. The other method of detection is watching for a planet to pass in front of it's sun. Since that requires a lot of things to happen in our favor (orbital position, planet size / speed) we're going to find a lot more planets by looking for the wobble.
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It's an observational bias, based on how they find these planets. If Neptune were as close to the sun as Venus or Mercury it would have more pull on the sun and create the "wobble" that scientists use to detect the majority of exo-planets they've found. The other method of detection is watching for a planet to pass in front of it's sun. Since that requires a lot of things to happen in our favor (orbital position, planet size / speed) we're going to find a lot more planets by looking for the wobble.