GameSpot has published a news article with a few choice quotes from a statement made by EA's head of global e-commerce, David DeMartini, on why some of their titles are not available on Steam anymore:

DeMartini stressed that EA will allow its titles to appear on any downloadable storefront, but there is a catch. The executive said, "We take direct responsibility for providing patches, updates, additional content, and other services to our players," and so insist on being allowed to "establish an ongoing relationship" with customers and contact them to inform them of new patches and available content.

"Unfortunately, if we're not allowed to manage this experience directly and establish a relationship with you, it disrupts our ability to provide the support you expect and deserve," DeMartini said. "At present, there is only one download service that will not allow this relationship. This is not our choice, and unfortunately it is their customer base that is most impacted by this decision. We are working diligently to find a mutually agreeable solution."