Because it is not already unbalanced with the God perks from Broken steel:
<blockquote>"In an email interview with Jeff Gardiner regarding August 3rd's release of the next "Fallout 3" DLC, "Mothership Zeta," I asked if it would even be possible to increase the level cap from its current 30 (as set by "Broken Steel.") Specifically, if the stat caps were boosted beyond 10 and skill caps were boosted beyond 100, would the game system simply break?"</blockquote>Link: Bethesda: Further Level Cap Increases Could Unbalance 'Fallout 3'
<blockquote>"In an email interview with Jeff Gardiner regarding August 3rd's release of the next "Fallout 3" DLC, "Mothership Zeta," I asked if it would even be possible to increase the level cap from its current 30 (as set by "Broken Steel.") Specifically, if the stat caps were boosted beyond 10 and skill caps were boosted beyond 100, would the game system simply break?"</blockquote>Link: Bethesda: Further Level Cap Increases Could Unbalance 'Fallout 3'