Why is Sunken Hollow a rare?
The land is a rare for a couple reasons:


It is a land that can provide two colors and it has the chance to enter untapped. Entering untapped is important for maintaining tempo.
It is a dual land*, in the classic sense of having two basic land types(Island Swamp). Lands with two basic types are very important, especially when fetch lands are currently in standard. A Plain-Island fetchland could get a Sunken Hollow, allowing the player access to black mana. 


*I call it a dual land, but there are many lands that can be considered dual. Any land that produces two (or more) types of mana may be considered dual, even if they aren't a basic land type (e.g. Llanowar Wastes produces green and black, but isn't a forest nor a swamp, and costs life to make colored mana). Many dual lands enter tapped, which trades mana fixing for speed. Some do give you a slight benefit (1 life, scry 1, relevant land type).  

And of the dual lands with two land types, each of them is slightly different. The original dual lands always entered untapped. Shock lands enter tapped unless you pay 2 life but it can be untapped the first or second turn. The dual land you have won't normally enter untapped until the 3rd turn or later.