The post has an answer to why that happened:

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.