Why exotic stylus not used on MM?

You will find EVERY tip profile on MM/MI cartridges that you find on MC.  No exceptions. Same for exotic cantilever materials.
 

The OP here isn’t very familiar with tip geometries and cartridges.  

Sumiko Amethyst is a Line-Contact design and MM.  You can also get AT VM95ML and AT VM95SH with MicroLine and Shibata styli, respectively.

Lots of choices for styli in the MM range.

@lewm  If you're looking for an mmc1 contact SoundSmith. I know they rebuild them and may even make new ones.

MMC cartridges are awfully old by now. Their suspensions are known for getting stiff over time.

Are you having them rebuilt?

Soundsmith's SMMC1 has a line contact diamond on a solid ruby rod cantilever. Is that too pedestrian or is the Sapphire tube that much better?

There are not any Exotic Styli on offer at present, especially as outlined in the list above.

These are no longer mysteriously unique or unusual, most who concern themselves with them will be able to recite the unique dimension for each type, whether it is an artificial grown raw material, as well as where to acquire one or all listed.

One Styli that is possibly closer to the used Terminology 'Exotic' is the Swiss Produced 'Black Diamond'. The part has certainly being assessed for being a suitable replacement part for the above listed Styli.

It is early days yet for this part, so it will be interesting to see how differing dimensions and refinements are produced for the part .