Bitcoin blockchain is one of the most fundamental building blocks to blockchain technology. It was originally designed only to record transactions and nothing else. Scripting language which bitcoin blockchain supports is stack based which does not provide much help to developers to built complex applications on top of it. There where many altcoins developed like Namecoin, Mastercoin and etc, but the script was not compatible to write complex contracts.

In Vitalik words:

I was working on Mastercoin and colored coins back in 2013; after a few weeks working on the projects I realized how the approach of trying to target a specific set of use cases was too limited, and started trying to make a blockchain that supported more and more general forms of computation. After a few iterations, the idea that is now called Ethereum, at least in a very early form, emerged.

Ethereum was first of it's kind, a blockchain with built-in Turing complete programming language, which empowers developers to write complex smart contracts on top of it.

For better reference you should watch Vitalik early presentation of Ethereum.

PS. None of it could have been achieved, without the invent to Bitcoin blockchain by Satoshi Nakamoto.