Why does TreePlot ignore the vertex labels in a TreeGraph argument?

TreePlot predates Graph. Only in version 10 was it updated to handle Graph according to the documentation. Since Graph supports more advanced functionality than TreePlot it is unsurprising that only a subset is supported by the latter, and VertexLabels is not supported. If you convert the Graph to an older, more basic format TreePlot will work:

TreePlot[Rule @@@ EdgeList[aTree], VertexLabeling -> True]

Yes, it would be nice if this were handled automatically. However other parts of the system that are in greater need of attention (IMO) still await update, so again I am not suprised.

An update for the record. The same situation described by Mr. Wizard appears to be the case in version 11.2. However, using Graph with GraphLayout->"LayeredEmbedding" can achieve the desired result.