Why haven't Blu-ray discs made DVDs obsolete?

They have, if you can't see the difference in quality then there's something wrong especially on an LCD dvd's look like crap unless you are watching them on a CRT screen because DVD movies are natively in 480i and most LCDs and players have terrible de-interlacing and upscaling quality.

I've professionally compared the difference between blue ray technology and dVd, and let me tell you this, there is no difference between them as far a quality is concerned. Blu Ray is just a marketing term.

Likely because no one wants to keep replacing their **** over and over again if it works. 

It's called the 'Good Enough Factor' . Many people just don't care if blu rays are better.DVD is good enough for them.Also the 1080p upconverters on dvd players have become very very good.

Blu rays are more expensive and some people don't need them. I have still to make the jump to a 4k TV (on my list of things to do when I can can be bothered). I haven't bought a dvd in years due to streaming.

A lot of people went from DVD to streaming.  If I went to a 4k TV I would just stream and not bother with physical discs. blu ray is something that I just have never had a need for and will probably never have a need for.

For the same movie or series box set, BluRay costs more than DVD.

Plus, to play BluRay, you need to buy a BluRay player.

Not everyone wants to incur those higher costs.

And then there's also 4K TVs, 4K BluRay players, and 4K BluRay discs. Also, all of those cost more than their non 4K equivalents.

4k and Streaming have made both obsolete.

Superior formats do not automatically eliminate those that are inferior. 

The persistence of the demand for new vinyl records is proof of that. 

BDs are more costly to manufacture --- and therefore to buy --- than DVDs,  

and far fewer people than you might think have invested in BD players. 

I bought my first BD player a few weeks ago, not from a store, 

but at an estate sale, for $5. Otherwise I would not have one.