Welcome to the site! I'll try and give the general answer you're looking for:
When NIST ran the AES competition in 1997 - 2000 to select the best symmetric cipher, they were looking for an algorithm that was well-balanced across a range of uses. The winner was the Rijndael cipher, which we now simply call AES.
AES: the Advanced Encryption Standard
The goal of the Advanced Encryption Standard (AES) competition was to specify "an unclassified, publicly disclosed encryption algorithm capable of protecting sensitive government information well into the next century". The AES competition was organized by the United States National Institute of Standards and Technology (NIST). [source: cr.py.to group]
The cr.py.to page lists the complex criteria that NIST was looking for in choosing a winner; it includes things like CPU and memory usage, easy to implement in both hardware and software, soundness of the mathematical theory behind it, etc. Rijndael was chosen because it scored the best across all categories.
If you checkout the wikipedia page for the AES competition, you'll see this:
All five algorithms, commonly referred to as "AES finalists", were designed by cryptographers considered well-known and respected in the community. The AES2 conference votes were as follows:
Rijndael: 86 positive, 10 negative
Serpent: 59 positive, 7 negative
Twofish: 31 positive, 21 negative
RC6: 23 positive, 37 negative
MARS: 13 positive, 84 negative
As you can see, Serpent was a close second, and Twofish also had net positive votes. That doesn't mean that Serpent and Twofish are bad, just that Rijndael was a better fit for the complex criteria that NIST was looking for.