Image quality and build.
The 70-300 VR is comparable to the 70-200 f/2.8 VR over the same range in common apertures, though there are some differences. The 55-200 is good, but really needs to be stopped way down (f/11, usually) and at that point you pretty much need direct sunlight and/or high ISOs to get anything useable with it.
The 70-300 is also built better: it's a semi-pro lens, not a consumer lens: metal mount, tougher body, some features the 55-200 doesnt gave (focus scale, focus ring, etc).
It's kinda like comparing a Hummer to a Jeep. Both will go off-road and do a decent job, but a proper humvee will do it more easily and effectively.
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