SMP, NUMA, local memory - limits on numer of processors
When each processor has its own memory, you end up with separate computers in a network. This is limited by the network speed, because you still need to distribute work packages and collect results.

SMP is limited by memory bandwidth. WHen your processor cores compete for memory access, they end up waiting for more and more time, so even if you can add more cores, the total amount of work that can be performed will not rise after some point. In addition, the length of the connection between the cores limits the bus clock.

NUMA is just a specialized network that emulates access to non-local memory, so it still has the same limitations as a regular networked approach, and IIRC the protocol requires direct pairwise connections between cores, so this becomes a signal routing problem as well.