[JeremyReimer]

They weren't owned by Activision-- Activision was only the publisher for Destiny, and never acquired the studio. When the publishing agreement came up for renewal, Bungie and Activision agreed to go their separate ways, for their own reasons. It's likely that Bungie wanted more control and more independence from the partnership.

Bungie's aversion to being owned stems from their Microsoft days, when they yearned to be free from having to make Halo sequels.

However, the reality is that game development is expensive. Destiny doesn't have a monthly subscription fee, so the income for the studio drops dramatically between paid content expansions. It's tough to keep that going without outside support, given the cost of creating said content.

(Obligatory disclaimer: I work for a division of Activision, but these opinions are solely my own)