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Why would a designer move to a multicore organization rather than increase parallelism within a single processor.

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In the case of pipelining, simple 3-stage pipelines were replaced by pipelines with 5 stages, and then many more stages, with some implementations having over a dozen stages. There is a practical limit to how far this trend can be taken, because with more stages, there is the need for more logic, more interconnections, and more control signals. 

With superscalar organization, performance increases can be achieved by increasing the number of parallel pipelines. Again, there are diminishing returns as the number of pipelines increases. More logic is required to manage hazards and to stage instruction resources. Eventually, a single thread of execution reaches the point where hazards and resource dependencies prevent the full use of the multiple pipelines available. This same point of diminishing returns is reached with SMT, as the complexity of managing multiple threads over a set of pipelines limits the number of threads and number of pipelines that can be effectively utilized.

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