I think it is dependent on the one supported by the system. Compatibility issues would also be one of the major parts. Other exchanges that are programmed to use legacy addresses may find it hard to transition to segwit because of security risks when in transition. Since there is a need to update/upgrade stuff, other exchanges left it be until they are confident enough that there will be no vulnerability instance that may happen during the transition or they have enough funds to hire a competent developer. Remember exchanges are not like having a personal wallet updating from a legacy address to segwit address, it is somehow complex and if their tech team is not competent enough, they might suffer losses during the update either via glitches or operation delay.
Thank you for your comprehensive explanation of the risks and potential causes. I understand your concerns, of course, and I understand exchanges' concerns about major problems and losses, or vulnerabilities and malfunctions during the switch to new addresses.
However, on the other hand, this switch is necessary for the safety and continued operation of the network. Private interests are often sacrificed for the public good. I mean, the safety and continued operation of the network are more important than the losses or problems that exchanges may encounter. There are major exchanges that have already taken this step, and I haven't heard of any problems occurring.