Why develop new stream ciphers when block ciphers can be trivally made into streams?
The biggest reason is performance. Stream ciphers are generally faster than block ciphers and perform fewer operations. Stream ciphers only need to generate a pseudorandom output while block ciphers need to be pseudorandom permutations. So when you create a steam cipher out of a block cipher, you are doing a lot more work to get the same effect. To get an idea, look at how much more work is required to encrypt 16 bytes with AES than RC4.