a new hope - Why does the Star Wars universe use such a primitive technology as data-tapes?

In A New Hope, Admiral Motti says that the Death Star plans were stored on data-tapes.

“Don't try to frighten us with your sorcerer's ways, Lord Vader. Your sad devotion to that ancient religion has not helped you conjure up the stolen data-tapes”*

I always figured this was something that would just be conveniently ignored in future installments, but Rogue One doubles down on this by making it clear that the Scarif facility is storing the Death Star plans on a data-tape.

Given that holograms, droids, datapads, and other advanced electronics exist, why does the galaxy far, far away use such as primitive technology as data-tapes at all, especially for something as important as the Death Star plans (which can be transferred to whatever un-tapelike thing the rebels hand off to Leia)?

Irregular Webcomic #67 by David Morgan-Mar, licensed under CC by NC-SA 3.0, (C) 2002-2017

_{* Just realized that Motti’s really rubbing salt on the wound since the stolen data-tapes were almost certainly destroyed by the Death Star’s destruction of Scarif and Vader surely knows that.}

Answer

Because it's the backup and backups are better on tape.

The fewer moving parts an object has, the more robust it is. Additionally, the long term degradation of electronic memory cells is a problem. So when you make a back up of something, you want it to be able to survive a long time. Tape can do this.

Technology in Star Wars has advanced enough to create a 512-million exanode capacity monomolecular-switching binary tape the size of a book. In English, that means they are using nanotechnology to encode data into the tape at the molecular level.

Remember, this is the backup, it's not the version that will be actively accessed, it doesn't need to be quick like flash memory is (that said, it is still pretty fast in real life).

And finally, it is much (much) cheaper. In real life, hard disk drives (HDDs) cost about 50% more per GB than tape, and solid state drives (SSDs), which are far more robust than HDDs, are a whopping 850% more expensive per GB than tape.

For a real world example, Google made headlines in 2011 when it was discovered that they kept long term backups on tape. So it's not a bad method of data storage at all.

In short, tape is the superior backup method because it is cheaper, has high capacity, greater reliability, and better longevity.