Why DVDs are still popular

1. People weren't ready to move on from DVD to Blu-ray. DVDs started to be used for Films between 1995 and 1999, depending on where in the world you live. Blu-ray was introduced by 2006. Many people by this point, had started to build up a large collection of films, and IMO felt rather annoyed by technology moving on to a new disk format so quickly.
2. Between 2006-2008 there was a small battle between Blu-ray and HD DVD (mainly fought between the Playstation 3 and the XBOX 360), to which format would be THE high definition format to prevail. Many people, outside of console gamers, were reluctant to purchase a dedicated Blu-ray or HD DVD player due to uncertainty over which would win. Nobody wanted to be left with useless technology and film collection they would be unable to use.
3. The difference in definition between Blu-ray and DVD isn't all that great, to the average viewer. Especially when viewed on smaller TVs. It does't improve the enjoyment of a film for most people, especially when faced with the prospect of re-purchasing your back catalogue in a new format, which, incidentally, will also cost more than the DVD version.
4. By this point, many people also had MULTIPLE DVD players in the home. Most people would be reluctant to replace all these. Even if you brought one Blu-ray player for the living room, when purchasing a new film, would you buy one which you can view on ONE of your pieces of tech, or ALL of them?
5. Steaming technology harms Blu-rays chances of replacing DVDs further. Most people who avoided using Blu-ray in the early days, suddenly saw yet another change in how media content would be delivered to us at home. It made more sense to just start consuming SOME content on-line and the rest through their existing DVDs
6. LEGAL streaming services actually have quite a limited number of titles available. New releases are often delayed compared to the physical media releases. Combine this with the above points, and you're left with quite a number of people who will be purchasing these films on DVD.
7. In my experience, finding older, and more obscure/world cinema, is often hard, or impossible on Blu-ray or through streaming services, meaning that its easier, or in some cases, only available on DVD.
8. Purchasing a Blu-ray player, now, seems a little pointless if you don't already own one, since streaming will only improve and increase in popularity in the coming years. But...
9. Many people still like owning a physical disk. 
10.  I have seen on multiple occasions where a friend will put in a Blu-Ray disc only to be told that the Blu-ray player needs a firmware upgrade to play the movie.