Flash takes too long to load
Viewers of your website will not be patient enough to wait for your website to load. Flash websites can take longer to load, which could increase your visitors bounce rate. Since loading time is a critical factor, it is key t o opt for a fast loading technology such as HTML 5.
Flash Limited Mobile Phone/Mobile Device Viewing - Most mobile web browsers don’t support Flash and can’t display Flash websites at all. More and more people are doing much of their browsing on mobile devices and you don't want your website to be one of the ones they can't pull up.
Flash is bad for Search Engines
If you wish to increase traffic to your website by making use of search engine optimization techniques, then Flash is not an option for you. Search engines cannot index Flash pages correctly as obviously they cannot read the content. The look and feel of your website is not a factor for search engines while indexing. As Flash pages do not have any content that contains specific keywords, those pages fail in reaching top spots in search engine results. In fact, Flash can sometimes decrease your search engine rankings. Many search engine algorithms have been redesigned in such a way that websites which have no direct relation between meta data and actual content are getting blacklisted as search engines assume that such websites are trying to gain traffic by using deceiving techniques. In case of Flash websites, as there is no content, and only a few keywords, search engines could potentially consider the websites as being deceptive.
Flash consumes more bandwidth
People who use slow Internet connections will feel a noticeable difference in browsing Flash based websites; other processes or applications become drastically slow as Flash based websites consume more bandwidth. This will make them stop browsing your website. Though high speed Internet connections are widely available, you need to target all kinds of viewers while building a website.
Flash creates restrictions on Usability
Most people use the Back button frequently to go back to the previous page they were surfing. But in Flash, the Back button is disabled by default. In such cases, there are more chances for viewers to click Close and end the browsing session on your website. Moreover, if Flash is not already installed on the computer of the viewer, they will have issues in viewing a Flash based website. They will be provided a link to download Flash, but many people will not bother to do so. Thus you might lose many of your potential visitors.