Why don't we have vehicles with the speed of airplanes, the low capital and operational cost of motorcycles, the cargo capacity of trains, the cabin space of luxury yachts?
NoSQL is just a brand name for a loosely defined set of engineering trade-offs. It's easier to achieve the scalability and fault tolerance needed for a lot of modern Internet systems if you don't also require ACID and the huge weight of features and old usage patterns that the market expects of a relational (which they read as "SQL") database. If somebody wants to pour $10-50 million into making an open source NoSQL-scale RDBMS, great, but some projects are too complex for any but the most robust open source communities to maintain, never mind create.
There's really two dimensions: relational and ACID. The relational aspect is the R in RDBMS: if you have relational data, you can interact with it in mathematically defined ways. The ACID part is the MS: in the past, when we wanted to store and ask questions of our data, we also wanted it managed such that we had guarantees on it.
Depending on the project, NoSQL solutions can relax both of those dimensions. I typically refer to "NoSQL" as non-relational databases.
NoSQL is just a brand name for a loosely defined set of engineering trade-offs. It's easier to achieve the scalability and fault tolerance needed for a lot of modern Internet systems if you don't also require ACID and the huge weight of features and old usage patterns that the market expects of a relational (which they read as "SQL") database. If somebody wants to pour $10-50 million into making an open source NoSQL-scale RDBMS, great, but some projects are too complex for any but the most robust open source communities to maintain, never mind create.