A lack of diversity doesn't just harm the growth of a scientific field by limiting alternating viewpoints - but also by undermining research already carried out.
WEIRD (Western, Educated, Industrialised, Rich and Democratic) a term coined to indicate the issue raised by the practice of a number of researchers.
It's quite common practice to employ students in order to complete experiments and research, however, this has a few drawbacks.
These students are often not reflective of the general population, and as such while the research may be useful may have a lower ecological validity than does research that utilises a larger subset of the population.
Imagine for a moment you wanted to explore Internet usage or typing speeds - if working for a university it would seem only natural to use a selection of students.
However, the typing speeds of students may not be reflective of the population as a whole or the Internet usage.
If psychology is the study of the human mind and behaviour, then it's imperative that researchers widen the fishing nets of selection to include a wider section of the population rather than just the WEIRD ones.