Why would I want to use a non-relational database?
Take a look at the CAP Theorem
And the PACELC interpretation
Relational databases tend to make one set of trade-offs, and non-relational tend to make a different set of trade-offs. For massive distributed datasets, non-relational sometimes makes more sense.
There is also a sense in which non-relational databases can eliminate a lot of the ORM pain, but again there are always tradeoffs. In some use cases, non-relational storage can be faster, because all the data for a particular hierarchy can be stored closer together on the disk. Also note that non-relational databases do still have query capabilities.
In the end, it's about making the appropriate set of trade-offs for your particular use-case.
Look at Nosql : http://en.wikipedia.org/wiki/NoSQL
It's basically because of scalibility and performance.