A selection of views on the topic of senior scientists turning down review requests (D. Graur Nature 505, 483; 2014).

Paul Flicek says:

Graur makes the anecdotal observation that those scientists who publish most often are more likely to turn down requests to review papers. But manuscripts are only a part of a senior scientist's peer-reviewing activities: these include peer review for individuals and institutions, and of many more grant applications than they themselves submit.

S. Sudha Tushara and S. Sudarsan say:

A reviewing 'impact factor' awarded to reviewers per paper would entice reviewers, and help editorial boards and funding agencies.

Bob O'Hara says:

Perhaps scientists who publish more are asked to review more, and thus even if they review as many manuscripts as everyone else, they will still appear to be less cooperative because they decline more manuscripts.

S. A. Luis, M. Madadin and R. G. Menezes say:

Nominal financial incentives would help too, as would mandating involvement with peer review as a prerequisite for academic promotion.

Samad E. J. Golzari says:

The quality of a mandatory peer review might not be the same as a review undertaken voluntarily.