How can I measure why users never return to a web app?

I'm working on an application that's been in use for the past 3 years. It's a web-based product and we have been tracking its usage using Google analytics for the past 6 months (since the time I joined). Now I've been tasked to help them improve the product. I'm not sure where to start.

But one data point, that's glaring is - 90% of the users don't log into the application after the first time. The hypothesis is users don't find anything interesting to login again. Are there ways to probe this? I was thinking of usability study and probably sending out surveys. Request insights from your experience on how best to handle this? If usability study and surveys are the way to go - how do I focus on this?