Q: Why has the editor sent me only one reviewer's comments?

There is no reason why the editor would not send you the comments of all the reviewers. This makes me think that, possibly, your paper was reviewed by only one reviewer. Although journals usually send a paper to at least two reviewers, sometimes, they have to make do with one reviewer in case they have been unable to get another reviewer. In such cases, the editor himself/herself also does a fairly detailed evaluation of the manuscript to compensate for the lack of another reviewer. However, you can consider writing to the editor to confirm this.