Silverlight has a lot of benefits, but as a new technology, it also has problems. As a new technology, it is inevitably riskier as many of the kinks haven't been worked out yet. Managers, who want to avoid unnecessary risk, may shy away from such a technology. However, there are ways to encourage a manager to at least consider Silverlight:
Show an actual demo of what Silverlight can do (such as on the gallery). Talk is cheap, but seeing Silverlight in action is powerful.
Where feasible, consider developing simple internal tools with Silverlight. Managers almost expect devs to always insist on using the latest technology, regardless of it's business value. But if you believe enough in the tech to invest your own time learning it and applying it to a simple business problem that your department faces - that carries a lot of weight.
Emphasize the aspects of Silverlight that would benefit your team - perhaps a rich UI with animating charts, or drag and drop, or rich media, or C# on the client, or cross-browser, etc...
If all else fails, consider a little fear-mongering: "Our competitors will be using this". If not Silverlight, at least a Silverlight-competitor like flash.
Some managers were hesitant when JS came out ("it's got cross-browser problems", "not all client support it"), when .Net came out ("J2EE is the established enterprise platform"), when Ajax came out ("it will have security holes"), etc... There's understandably going to be some skepticism with Silverlight too, but that's ok. I personally believe that Silverlight can deliver, and therefore instead of trying to encourage managers to adopt it, managers will be recruiting developers who know it.