Millions of dollars were spent by various companies to build application specific integrated circuit (ASIC) computers in an attempt to monopolize mining. These devices made it so GPUs and CPUs were no longer useful in the mining process. As a result, the vast majority of Bitcoin mining and transactions would now be processed by large data centers that required hundreds of thousands, or millions of dollars in investment.
Now that Bitcoin computational power is handled almost entirely by large data centers, the currency has rapidly changed from a distributed, decentralized currency, to one that is much more centralized and vulnerable. Companies such as Cex.io are routinely hitting 40% PoW computational power on a daily basis, and will soon be able to perform 51% attacks at will, if they so choose.
Due to the problems ASIC chips brought on the digital currency platform, a new currency, Litecoin was created. Litecoin attempted to utilize the Scrypt hashing algorithm, which is more memory intensive than SHA256, to deter ASIC use. As of January 2014, there are now Scrypt ASIC chips being deployed by the Chinese for mining. So far, around 1000 Mhash of Gridseed ASIC chips have been brought online and used on the litecoin and middlecoin.com network.
The Chinese chips have not been released on the greater market at large, but eventually they will, and the same problems that SHA256 currency faces will come to the Litecoin and other Scrypt currency networks next.