I suppose I should elaborate a bit:
The idea is that the "best" way of establishing an informed opinion is by formulating (or adopting) the theory that fits the most data using the fewest number of assumptions. That being said, "hard solipsism" has an extra assumption, one that is not necessary. That being that that which we observe, external reality, is not all there is. It's simply added on to our existing understanding, but for no real reason, with no justification. It explains no more data. In fact, it might even be impossible to create a justified theory that adds up to something being outside of our reality.
On the other side, while "soft solipsists" hold that we can never be certain reality exists, I hold that the assumption is...well, it's the only way things could really be. To say that there could be something outside our reality is incoherent, and here's why:
The concepts of "being", of "existence", of "outside". everything applies within reality. To ask whether reality really exists or is simulated is to commit a fallacy: existence applies within reality. You cannot apply part of the whole to the whole itself. It was like asking what was "before time", or asking if there is something "outside space". It doesn't make any sense.
Furthermore, to me the whole solipsism thing is really a clear case of Russell's teapot. The possibility of all that we experience being a simulation is really as irrelevant as the possibility of the teapot floating in space. Our experience is properly basic, and that's the starting point. Anything believed above that must be justified, and, as previously stated, to be justified, the theory must explain the most data with the fewest assumptions. Solipsism doesn't do that, hard solipsism even more so. I think.