It's a good question. The answer has to do with the properties of currency itself. Try a thought experiment: Say you go to the grocery store and the cashier tells you your total is 565 Goblats. You look in your wallet or on your phone and see that of the 1,000 currencies you have, including Bitcoin, Litecoin, Ether etc. you don't have any Goblats. What do you do? Try to find somebody to make an exchange? All you want to do is pay and leave.
On the other hand say the merchant doesn't want to miss any sales so he accepts 5,000 currencies. How does he manage the exchange rate? Some coins may lose 40% in one day or even one hour!
See the problem? For currency, people (average people, not traders) only want very few choices, not many as it makes things too complicated.
I see the problem with there being 5,000 currencies. I don't see the problem with there being 3-10. Bitcoin for your savings, an altcoin for your every day purchases, an altcoin for investing with, etc... Also with proper wallet software the user wouldn't have to think twice about how to pay. Even if there were 5,000 options.