Is there a strong reason for Nullable's alias get this?

[FeepingCreature]

Addendum: The general principle is that a "narrowing conversion" 
should never be implicit. An example of a narrowing conversion is 
float to int, or int to short. In the case of integer types, D 
already follows this rule.

Nullable!T to T is a narrowing conversion, since T cannot express 
the null case. Therefor, it must not be implicit.