Mostly it is about price. Drum brakes require the least maintenance of all brake types and have lower initial cost than disk brakes. They are also not affected by damage to rims or discs and work in rain just as well as in dry weather (i.e. not effective, but no sudden loss of braking either).

The low braking power is more a benefit than problem for bike share bikes. These bikes are not expected to be ridden fast enough to need powerful brakes, but an inexperienced rider can throw themselves over the bars with the front brake and poor brake reduces this risk.

Mostly it is about maintenance costs and reliability. Drum brakes require the least maintenance of all brake types and have lower initial cost than disk brakes. They are also not affected by damage to rims or discs and work in rain just as well as in dry weather (i.e. not effective, but no sudden loss of braking either).

The low braking power is more a benefit than problem for bike share bikes. These bikes are not expected to be ridden fast enough to need powerful brakes, but an inexperienced rider can throw themselves over the bars with the front brake and poor brake reduces this risk.