CAP Theorem:
Consistency, Availability and (network) Partition Tolerance: pick two.
RDBMS are excellent at providing consistency and availability with low latency. Yet imagine building a massive parallel web map/crawling infrastructure on top of an RDMBS -- particularly with multiple datacenters and high latency links between these datacenters: it may be feasible for an enterprise/intranet crawler, but not for something at the scale of Yahoo or Google search (both of which build up their datasets/indices using map/reduce and then distribute these indices to search nodes for low latency querying via more traditional approaches).
CAP Theorem: Consistency, Availability and (network) Partition Tolerance: pick two.
RDBMS are excellent at providing consistency and availability with low latency. Yet imagine building a massive parallel web map/crawling infrastructure on top of an RDMBS -- particularly with multiple datacenters and high latency links between these datacenters: it may be feasible for an enterprise/intranet crawler, but not for something at the scale of Yahoo or Google search (both of which build up their datasets/indices using map/reduce and then distribute these indices to search nodes for low latency querying via more traditional approaches).