>> why are we -- as a community -- opposed to calling our software "open source" if we use the Elastic license?
The term "Open Source" was created by a group that later became the Open Source Initiative: https://opensource.org/history
By definition, a software license is not "Open Source" unless it meets all the criteria of the Open Source Definition: https://opensource.org/osd
The Elastic license does not meet point six of the Open Source Definition:
"6. No Discrimination Against Fields of Endeavor
The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research."
Therefore the Elastic license is not an Open Source license.
It's your software. License it how you want.