There are a couple of reasons for this. First, because blockchain is still a relatively new technology, there are not yet many widely-accepted standards for how it should be used. This means that different blockchain-based applications can end up using different types of cryptocurrency.
Second, because blockchain is decentralized, each application can choose to use whatever type of cryptocurrency it wants. This allows for more flexibility and experimentation, which is good for the overall development of the technology.
Right now, it’s probably best to think of blockchain as being like the internet in the early 1990s. There are a lot of different ideas about how it should be used, and so different applications are using different types of cryptocurrency. In time, as blockchain technology matures, we may see more widespread use of a single type of cryptocurrency. Or, we may see different types of cryptocurrency being used for different purposes. Only time will tell.
Other related questions:
Q: Why are there multiple crypto wallets?
A: There are multiple crypto wallets because different cryptocurrencies have different wallets. For example, Bitcoin has a different wallet than Ethereum.
Q: Can one wallet hold multiple cryptocurrencies?
A: Yes, you can use one wallet to hold multiple cryptocurrencies.
Q: Does each crypto coin have its own blockchain?
A: Yes, each cryptocurrency has its own blockchain.
Q: How many coins are on blockchain?
A: There is no definitive answer to this question as the number of coins on the blockchain is constantly changing as new transactions are processed and new blocks are added to the chain. However, according to CoinMarketCap.com, as of June 2018 there were over 16.7 million bitcoins in circulation.