They wanted to build an open, paid version of the Twitter API/ecosystem. But the value of the Twitter API lay in the access to their 100 million+ users, and all the valuable social data generated by those 100 million+ users. No amount of "open infrastructure", no matter how clever can ever replace that. It's like buying 1000 data centers in every country, and building an entire system that can handle a billion concurrent users, and saying "Yeah, we can take on Facebook with this", then realize the hard part was getting users, not scaling the infrastructure.