It's very easy for IT armchair experts to just say "use AES" because it is proven, but little consideration is given to the computing complexity of telecommunication systems.
We are not talking about a simple web or SSH server here, but network equipment that continually encrypt/decrypts thousands of sessions simultaneously. If complicated algorithms and keys are used, the processing power would be astronomical and a pratical implementation not feasible. Processing power of mobile devices also needs to be taken into consideration although they have increased massively, but the network side often gets forgotten.
UMTS also has network authentication, integrity protection and 128 bit keys, which is also a big improvement on GSM. There is also a stronger UMTS encryption algorithm that is based on the SNOW 3G cipher, but many devices don't support this.
Interesting in LTE, the two ciphers in the stardard initially are SNOW 3G and AES.