Because the plumbing is more complex.
I ran my two rads in parallel, but wouldn't do it again.
See, the flow restriction caused by rads relative to a modern restrictive block (Storm design, WhiteWater design) is bugger all.
Experiment here:
http://www.uitp.com.au/cgi-bin/approximator.htm
If you've already got split paths (eg: LRR WW outlet) and you've got two radiators, go for it.
A lot easier to fit two radiators in a case than one big one.