Why are home sellers giving concessions?

The main reason (in my area) is because home sales have slowed in comparison to the past few years. Price reductions and agent/buyer incentives are now part of the marketing. Sellers have reasons to move or sell (building a new house, moving to another area, relocation closer to family, divorce, death, etc) and in order to expedite a purchase contract (average days on market is in the months now) -- concessions/incentives are the first step towards enticing a Buyer to make an offer on their home.