Because it is literally a slower architecture per clock cycle:
http://www.anandtech.com/video/showdoc.aspx?i=3734&p=15
paragraphs 6, 7, 8
That was an assumption they had based on BENCHMARK performance on the product right at release.Not once have i seen an article that proved the 5000 series was slower on a per pixel shader basis.And for those saying its not the drivers then why do we have the 5770 slowly closing the gap on the 4870?You guys seem to forget the 4890 was just a overclocked 4870 so if the 5770 can get close to the 4870 then why couldn't the 5830 surpass the performance of the 4890 with driver updates?ATI are notoriously slow with getting driver updates out and the fact that they do it on a monthly basis makes things even worse.So its not out of the question to think they couldnt get better performance out of newer games.And worse performance in older games make sense since ATI has had years to get proper performance on tons of games for the 4000s series yet they have only had 5 months or so for the 5000 series.If someone can show me hard evidence that the 5000 series is gonna be slower than the 4000 no matter what then link away.If not then I'll wait a couple months and reserve judgment.