Drupal 6 is officially no longer supported by the Drupal community:
This was announced in 2015 and officially went into effect on February 24, 2016. It has been two years since Drupal 6 was supported and Drupal 7 may not be supported for much longer according to past release timelines.
Responsive layout is built-in.
It is very difficult to retrofit a responsive theme into Drupal 6. Drupal 8 is responsive out of the box and all community contributed themes have responsiveness built-in. On January 10, 2018, it became required by law for all federal sites to be mobile friendly and this is to be completed no later than 18 months after this was put in place (June 10, 2019).
Better performance.
Drupal 6 requires a lot of tweaking or external caching to achieve good performance. Many of these things are either built-in or easy to enable with Drupal 8. The latest version of Drupal has worked hard to improve the load time of pages, management of the content and many other aspects. Drupal 8 now works more efficiently, and hence it saves much time for the user in all circumstances.
Better security.
Drupal 6 no longer receives most security updates and it is not architected to handle modern security threats. Drupal 8 offers security best practices for writing code, generated PHP files, and configuration for site builders. Drupal also has a security team in place. If someone discovers vulnerabilities, patches can be quickly released. Stable modules that have a security review also have a shield icon to differentiate them from the rest.
Better customization: Mobile First
D8 has a much more robust plugin architecture that allows developers to customize almost anything. This even allows D8 to serve purely as a back-end CMS with a completely de-coupled front-end application which could include a mobile app. Drupal in and of itself is an open source content management system. This helps companies get faster with everything they do, execute at scale, and not be burdened by delays in bringing new ideas to market.
Better SEO
Drupal 6 is not optimized for modern search engines which can cause low page ranking. Drupal 8 is better out of the box and has a lot of community modules that can optimize it further. It’s increased site speed, responsive web design, mobile readiness, CAAS capabilities, etc. has made it better optimized overall.
Usability improvements.
Drupal 8 has significant usability improvements for managing content and users over Drupal 6. Throughout Drupal 8, the focus has been mobile-first and usability. All of Drupal 8’s built-in themes are responsive and built to be used wherever you may need to get work done, whether that be on your phone on the go or stationary on your computer of choice.
Enterprise application architecture.
Both Drupal 6 and Drupal 7 have problems separating configuration and business logic from the actual content. This creates a lot of challenges when trying to deploy a large or complex site. Flexible APIs such as actions and triggers, Database API, caching, roles and permissions, Form API, REST API etc. make Drupal 8 an ideal web framework, while continuing to manage the content.