Why are all ratings positive?
Everyvine rates vineyards based on the evidence available. Vineyard owners report to Everyvine their successes: harvests that went into an award winning wine or sold for high prices. For understandable reasons, vineyard owners do not report bad crops or bad wines.
Thus, the analysis Everyvine pursues is strictly to determine what successful vineyards have in common. Everyvine does not know of bad vineyards and therefore cannot assess what makes for bad grapes.
Additionally, not all vineyard owners provide information to Everyvine. There are undoubtedly many vineyards that are capable of growing high quality fruit which are not ranked by Everyvine. This is often the case when the vineyard owner has not provided any detailed information about their vineyard and no other vineyards with those site characteristics are known to be producing high quality grapes.