Why did my endodontic dentist refer me to another endodontic dentist?

It has nothing to do with upper vs lower, it has to do with variation in anatomy. Teeth are unique from person to person, just like fingerprints. That is why CSI folks can use dental records to identify people. Some roots, whether upper OR lower, are straight. These roots are easier to do than curved roots. SOME curves are SO extreme that only VERY skilled dentists can do them. Kudos to this dentist for knowing their limitations and making the referral. If a root canal treatment is not done well, you can lose the tooth. 

Typically, the most difficult tooth in the mouth to perform root canal therapy on is the upper first molar. This is because this tooth often has a difficult-to-find fourth canal, this tooth's roots are often curved making negotiating the end of the canal more challenging and the tooth is also situated near the maxillary sinus making healing of the root canal potentially more problematic. If one endodontist referred you to another endodontist it is probably because the second endodontist uses a microscope and/or CBCT imaging which will come in very handy in treating an upper first molar, especially through an existing crown. Kudos to the first doctor who recognized his limitations and made a wise referral so you can have the best chance at treatment success.Good luck!

It sounds like your tooth is complicated and no one wants to mess up! That's fortunate for you to have honest practitioners. I'd recommend a cone beam scan so the endodontist can see the hidden anatomy as it's a 3D view in cone beam.