First, AES-CTR isn't "similar to a stream cipher." It is a stream cipher. That means the real question is "why do we develop new stream ciphers when AES-CTR provides an acceptable one?"
The answer is that newer stream ciphers tend to be superior to AES in some way or another. AES is a secure cipher, but it has some bad properties; for instance, it's hard and slowish to securely implement AES in software (the design works great if you're building dedicated hardware and OK for software, but the most natural software implementations are vulnerable to side-channel attacks). Salsa20 is designed for easy and highly efficient software implementation, with many fewer "gotchas" that lead to side-channel attacks.
Also, there are inherent advantages to having at least a few unrelated standard algorithms, because it reduces the impact of a break of one algorithm. If AES is the only cipher out there, and AES is broken, the results are catastrophic, because not only is lots of previously encrypted data at risk, but there isn't a widely accepted alternative. With multiple alternatives, if AES is broken it's still a huge deal, but there are secure things to switch to. That's literally the primary reason why the SHA-3 contest happened -- SHA-2 was strong, but it was the only fairly standard strong hash, and NIST wanted an alternative if it was broken.