Not easy to only choose one reason !
If I had to though, I would say : SEO
1. Are you positioned on the right keywords
2. Are you helping the visitors in anyway : solve and issue, provide a solution to something they have been struggling with, ect
#Inboundmarketing
If I did not have to choose - in the following order :
-SEO
-Potential technical issues : load time, broken links
-UX / UI : from the colors to CTA's to the depth of the website
-Relevance : Is the visitor finding what he/she has been looking for
-Forms length
That's all I can think of for now, hope it helps
Wix