[2.1] Background for why SCOs were made to be bundleable (support relationships with other StixObjects), but versioning properties were not explicitly added

[sr@ctin.us]

Fwd from: https://github.com/oasis-tcs/cti-stix2/issues/176

Please see the further comments already made in the issue by other CTI  
members for further identified variances.


[2.1] Background for why SCOs were made to be bundleable (support  
relationships with other StixObjects), but versioning properties were  
not explicitly added

in section 3.6 for Versioning is reads:

In STIX 2.1, SCOs do not explicitly have those three versioning  
properties. Therefore, a SCO cannot be versioned unless custom  
properties (discussed in section â11.1â)â are used. Producers who do  
this SHOULDâ use the property names âcreated_by_refâ, ârevokedâ,  
âcreatedâ, and âmodifiedâ.
What is the background on this? Why was SCOs made to support  
relationships with other objects ("outside of the cyber observable  
container"), but the rest of the typical common properties were not  
added?

Given this is shown as an option for producers (as per 3.6), what were  
the original reasons for the lack of adding those common props, and  
what are the implications?