F5 Silverline vs Imperva on Gartner Magic Quadrant: Why is F5 regressing while Imperva consolidates its leading position?

There are a number of factors that go into the ratings, ranging from product capabilities to sales/marketing/product development plans, revenue and many more. My perspective is that F5 tends to find traction where companies have already been using their ADCs offerings. The WAF is rated highly by customers, but my opinion is that F5 hasn't kept up with Akamai and Imperva in areas such as bot management capabilities that might be particularly relevant to your use case. Akamai in particular has focused a lot of R&D (and acquisition dollars) on security, and now have a variety of API protection, secure application access, and data security services to go along with their WAF, and that would likely be a factor in Gartner's scoring.