One of the reason is the competition where more cryptocurrencies are created (invented) every week or month and we see that every altcoin is having only few differences that could make them better than bitcoin and better than other altcoins.

To be only 1 cryptocurrency is not fair like we don't like to use only 1 currency all around the world because the price could be manipulated from the main institutions or if the main people who produce the cash could print more cash then the value of that currency could drop a lot

And it could affect the whole world economy which is not good, regarding to this there are many reasons why there are so many currencies and cryptocurrencies and I personally wouldn't like the only bitcoin without other altcoins which help the bitcoin too to have the actual price and value because many people have learnt bitcoin through altcoins.