Why does Tristan und Isolde require singers with especially powerful voices?

Tristan und Isolde requires singers with especially powerful voices because the opera features a vastly enlarged orchestra that serves as the driving force of the drama rather than as a mere accompaniment.1

In Richard Wagner's approach to the music drama, the sheer weight of the orchestral sound creates a dense musical texture that would submerge a voice using traditional techniques. Consequently, the singers must possess the vocal power necessary to penetrate this massive orchestral sound. This demand for increased volume and projection led to a departure from the earlier bel canto style, forcing singers to adopt new methods of "singing on resonance" to meet the unprecedented vocal exertions required by Wagner's scores.12