The short answer is that with Prolific you can use any survey/experimental software you'd like. Behavioural researchers (our primary customers) don't tend to use SurveyMonkey, but instead use Qualtrics, Gorilla, and custom experimental software which have many features important for behavioural research (e.g. randomisation and reaction time recording).

In addition, SurveyMonkey audience uses a survey exchange provider CINT [1] and as a result you don't have any transparency of where exactly the participants are coming from, how they're vetted, the user experience they're having, and you can't communicate directly with participants or retarget them for longitudinal studies. All of which is important for research and are features that Prolific provide.

[1] https://www.cint.com/press-release/surveymonkey-audience-exp...