If using a Detail Line and drawing a circle Revit doesn't much care for attaching a dimension to the "side" of it, in this case it is a conduit, when they are sketched that way. We can sketch a short straight line segment and place a dimension that references that instead.
If we create and use Detail Items for 2D versions of real things then we'll find the dimensions more accommodating. The advantage of being a component is it can be re-used, provide a variety of sizes as types and they can be tagged. Lines have never been quite the equal to components in Revit's world. I do think it is a bit silly that the dimensions don't work on a detail line used to create a circle. We can only dimension the diameter, radius or from center to another element.