It's totally possible not to have an ICO. I think the main reason to have one would be so that the coin owner can raise money to develop the coin further. It provides a much needed capital boost to new coins.

As most have said here Bitcoin is the most well known cryptocurrency. This is a big factor towards investment. Far more people have heard about Bitcoin than say Litecoin. For those just starting out in cryptocurrencies Bitcoin is also probably the easiest to invest in.