Maybe it's too complex to coordinate two transport operators and a payment system? Or the operators are not familiar with the topic, after all they only run trains and buses from A to B as per standard procedure. There's no mentioning about "make the ride convenient and pleasant for the commuters" :bsmilie:
Simple reason: no one - not the transport providers, not EZlink, the government, nor the commuters - willing to pay for the hardware and software modifications needs at the MRT stations and busses.