The application of the top rate for trust is a long-standing measure since 1972. The presumption behind that rate choice, as I understand it, is that typically trusts are used by sophisticated, higher net-worth taxpayers who would, if they earned the income directly, likely be paying that top tax rate. Deviating from that is the concession in the testamentary context.
On November 5th, 2014. See this statement in context.