Because cooling performance is, at least, marginally better when using the cooler outside ambient air to cool with rather than trying to use the somewhat warmer internal case air. The delta difference inside the case from using a front mounted cooler is, in most cases, not even measurable or if so, barely, so that is a non-issue for people who worry that it's going to raise temperatures in the case. So long as you have at least one rear exhaust fan and one top rear exhaust fan, which is generally about the MOST recommended configuration, you should have no worries about GPU or VRM temps at all.
However, that is not to say that a top mounted radiator cannot give you a satisfactory level of performance as well, it's just not likely to be AS good as when cooling using ambient air instead of internal case air.
The Cullinan V500 supports radiators up to 360mm in both the front and top locations.
Rosewill makes "ok" cases, but they are not top quality like some others and tend to lack some of the "feel" of quality as well as some features found on more premium cases.
How much are you willing to spend on a case?
Maybe the Cullinan is the right case for you, but maybe you could get a much better case for a few bucks more.