Could be for many reasons:

* The cost of acquisition is less than the cost of developing it themselves. * An aqui-hire. Microsoft wants to pick up the dev talent and skillsets at Accompli. * To prevent a competitor from getting them. Maybe there was other interest that would let a competitor move in on existing Microsoft products. * To continue to build its 'modern' image. There's a lot of additional branding to do if they do it themselves, buying Acompli gives them some nice press and puts the Microsoft brand in front of Acompli's users (which may not be Microsoft's traditional demographic).