Why only choose the observations on L1 band?

[Mil1ium]

Hi, @shaozu
I have a new question about GNSS observation selection.

Acording to the the code (in function filter_L1 and L1_freq ), if I understand this correctly, I guess you just choose observations on these bands:

1. GPS L1;
2. BDS B1;
3. GLONASS G1 and G2;
4. GALILEO E1.

Could you tell me why not choose observations on other bands, for example GPS L2, L5 and BDS B2?
BDS B2, for example, seems to have better performance from what I read online.