[EOSIO/eos] Why is min_activated_stake a fixed value of 150’000’000’0000 instead of 15%? (#5326)

Why is min_activated_stake a fixed value of 150’000’000’0000 instead of 15%? In order to limit the system token SYS circulation is 1 billion?(…/constracts/eosio.system/producer_pay.cpp )