Adobe is ceasing the updates and further distribution of Flash Player because, simply put, it is extremely inefficient, and chock full of security issues. Most websites have long since switched to HTML5, which is a far more efficient and secure coding option.
First off, it's more readily accessible from any modern browser, including on mobile. The issue of not having the right Flash Player version for a specific website and having it not load properly, is no more. HTML5 is a universal code now.
Second, it's faster. MUCH faster. HTML5 is a code that runs natively in your browser without requiring any plugins, and it's a very efficient code at that. It's faster and also less resource intensive.
Third, security. Because of the fact that it's not a separate, proprietary program that you have to install into your browser, like Flash, HTML5 is a lot less vulnerable to security holes. It's not so much as a program or product as it is a standard. It was actually defined as such by the been World Wide Web Committee, who terms and implements these standards for internet technologies and protocols. Hence why every modern browser now works natively with it, and why every website is now made to work with HTML5 as a default, no plugins required.
Most of us won't really see any impact from this turnover however, as the majority of the websites on the World Wide Web have already been converted over to HTML5. The few that haven't though, still have some time to convert to it. It really is just more efficient. The evolution of technology.