There isn't always a need to reinvent the wheel, even if you have the resources to do so; in this case Twitter isn't trying to compete on the technical function

There isn't always a need to reinvent the wheel, even if you have the resources to do so; in this case Twitter isn't trying to compete on the technical functionality of their blog - therefore it doesn't make business sense for them to over-invest resources into it - they are therefore effectively outsourcing it (the technology side, at least).