Good to know that all the major vendors are continuing to develop them though.
Web Components were a Google effort and little negotiation was made with other browsers before shipping. Like most negotiations in life, parties that don’t feel involved lack enthusiasm and tend not to agree.
Web Components were an ambitious proposal. Initial APIs were high-level and complex to implement (albeit for good reasons), which only added to contention and disagreement between vendors.
Google pushed forward, they sought feedback, gained community buy-in; but in hindsight, before other vendors shipped, usability was blocked.
Haven't we learned this lesson before?
I think it's more likely that Google didn't want other vendors' interference, to avoid design by committee thwarting whatever they were trying to achieve.
Refer to how SPDY was developed. Lessons were learned because stuff got done and it was out there. Then everybody came back to the table a little wiser. On the whole at least that's how it looked to me.