Because they are "real".   Dynavector Japan is a real company, not a marketing company.   Established in the 1970's by the physicist Dr Tomainari it really does design and manufacture moving coil cartridges and tonearms.   Dynavector invented specialised coil winding machines that can wind coils with the world's finest (and lightest) wire.   These machines are secret to this day!
Dynavector products are made in-house in Tokyo Japan and they have an extensive and ongoing Research and Development program.   No Dynavector products are made by outside companies.   Most other cartridges on the market are simply re-branded OEM units, all more-or-less identical inside with all the R&D going into fancy brochures and wild claims.
Dr Tominari discovered the cause of a harshness common to ALL cartridges when playing loud and complex musical passages.   Dr Tominari solved it with the invention of a new magnetic material (patented) that is now in all Dynavector cartridges.   It totally eliminates any harshness even in the midst of the most massive musical crescendo.  The sound is always smooth and crystal clear.   This solution is only found in Dynavector cartridges!
Hint:  Always audition audio equipment using large scale music.  I'll post a FAQ on how to audition audio gear FAST and sort out the "diamonds from the dross!"