Bitcoin currently accounts for 95% of the value, and volume of cryptocurrencies traded in the market. Bitcoin has managed to reign supreme over altcoins because it was the first cryptocurrency to exist, and initially attracted all of the mass media attention and adopters.
With digital currencies, network effect plays an important role: since Bitcoin has the highest adoption rate among other cryptocurrencies, it creates a virtuous cycle of investment and interest. More miners will invest in mining Bitcoin over other altcoins. With more miners in the network, Bitcoin is arguably more secure as an accurate public ledger, increasing the confidence in Bitcoin even further.
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Moreover, Venture Capital investments are targeted on improving Bitcoin technology as opposed to the other clone cryptocurrencies. With increasing financial and intellectual capital, a better product can accelerate from the rest.
Some notable projects in this space which I would keep an eye on are Maidsafe, Ethereum, NXT, Ripple and Counterparty. These are tangential to Bitcoin either adding much more functionality, or improving on the protocol, or building on top of the Bitcoin blockchain.