We prefer a status quo.
We don't like to change our behavior unless the incentive to do so is compelling enough. If the expected gain is too small or uncertain, we like to keep doing thing as we always did. Keep this in mind when writing or talking about the gains people will get from your product or service: make it a little extra convincing.
Recently I've seen some (often absolute) statements going around, generally in the line of "open source commerce platforms are a terrible idea". Now of course different solutions always have different pros and cons.
A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. I've created a version of this chart/pyramid applied to CRO which you can see below. It contains the options we have as optimizers and tools and methods we often use to gather data.