I think their end goal is relevance, not so much natural language relevance
as technical relevance. The emoji phenomenon has, probably more than any
other single factor, motivated the adoption and maintenance of proper
Unicode text support at both the platform and application layers.
Executives, managers, and engineers who once only paid lip service to I18N
support were feverishly concerned with supporting emoji, addressing alot of
lingering technical debt up and down the software stack. It was Apple and
Google who pushed Unicode to establish a process for incorporating emoji. As
a technical matter supporting emoji is first and foremost a matter of text
processing, which as a practical matter is for them equivalent to matters of
Unicode and UTF-8 processing. American companies were never invested in
Asian encodings like Shift JIS, which I think is where emoji first emerged.
The upshot is that American companies had more to lose from a fragmented
I18N ecosystem compared to Japanese companies more accustomed to such
complexity.
Where emoji goes so too go vendors and developers. If the Unicode Consortium
doesn't own emoji, there'll be a de facto fork of Unicode as a standard and
as an ecosystem. (Similar to WHATWG snatching HTML away from W3C.) Before
Unicode began officially incorporating emoji an industry standard was
emerging around glyphs assigned within the private use range. I think
Unicode is still making up for lost ground in that regard.