Because search criteria and the search strategy used will be part fof the Methods section of your published paper. It must be described in sufficient detail so that a reader could reproduce the search.
The decisions incorporated into your search criteria can increase or decrease the likelihood of getting your systematic review project published. Remember that the goal of any systematic review is to produce the highest possible evidence. Therefore, frivolous or arbitrary criteria can be criticized. A search strategy that is overlooking possible relevant sources or missing key concepts may be rejected.
The search criteria and strategy is intended to be comprehensive and retrieve ALL relevant studies while minimizing bias. This will retrieve A LOT of sources that will be narrowed down but the research team to only the few relevant studies.
And keep a record of where and how you searched. It will make your project reproducible and it will increase its credibility.
Search for clinical trials for results that are not statistically significant. They are less likely to be published as journal articles.
Search for conference presentations.
Be critical of the inclusion/exclusion criteria of your own project. Other researchers will be. Are you using the most appropriate criteria possible to maintain the high quality of the final product?