I imagine that at least one factor there is that building up is prohibited by zoning—a super brief glance at Concord's zoning map & code it looks like the only kind of residential buildings you can build anywhere without special permission are single-family.

Now there are surely people living there who would argue that this zoning has protected the shape and nature of the town they that they prefer, but the flip side of that coin is that, at $1.4m, a median home in Concord costs more than 3x that of the country overall.