Well first off, Disney doesn't "have to" let them expire, they are simply allowing them to, so you have to wonder why that is? but Universal is the most likely studio to acquire them because DreamWorks is basically Steven Spielberg's studio/production company, and he has some say as to who he want to do business with. Most of his "cinematic legacy" so to speak, Jaws, E.T., Jurassic Park, has been with Universal. But Spielberg also has ties to Disney with Indiana Jones, Roger Rabbit to some degree, and though his relationship Kathleen Kennedy, Spielberg got J.J Abrams the Star Wars directing job. So Spielberg is maybe the peacemaker in this situation between Disney and Universal, but neither one is going to give up something for nothing