It's superior performance and customer support. As a UNIX engineer, I tested the performance of PageSpeed. Their site was #1 because it is closest to my customer base.
Siteground provides the best hosting service, specialising in Wordpress sites, which is what I develop. Their assistance is second to none and in terms of uptime, I can't fault them.
We had very bad experience with our current service provider. Having troubles with e-mails and WordPress Hosting as they did not update PHP for OVER A YEAR, all they did is try to push us to a more expensive Virtual Hosting which they would take even less responsibility.
I run a small local SEO company. Most of my customers are on SiteGround. We've been recommending them for several years. 3 reasons: Site speed, Site Security, and Excellent 24/7 customer service.
They have great customer service with no wait time and they know just what to do quickly. For me it's important to know that I can reach someone quick if I need help with something on my website as I'm not all that techie.