PTM (sorted maps) are implemented with red-black trees. Transients (IEditableCollection) needs some way to O(1) switch from persistent to transient and vice-versa, and a faster than O(n log n) way (amortized) to collect values during transient batch updates.
This pre-dates my involvement on the Clojure team, but I presume that the PTM red-black tree implementation can't provide those performance guarantees. Or perhaps it it possible but was just too difficult to be worth doing. I assume it's the former (due to rebalancing), but can't say for certain.