As devices get smaller, more design and engineering tradeoffs are required. With a desktop PC, there's a large screen and it's easy to include expansive options and features without big tradeoffs. With a smartwatch there are lots of constraints: screen size, readability, weight, battery life, holding up one's arm for extended time periods, and so on. These constraints demand lots of tradeoffs. Apple excels at making design and engineering tradeoffs, meaning a smartwatch plays to the company's strengths. Apple specializes in: (1) simplifying products through careful design/curation; (2) making design and UI choices on behalf of users; and (3) filtering/managing the features and information provided to users. Algorithms and machine learning aren't as relevant here. Judgment, taste, and design prowess are required.
It will be interesting to see if smartwatch companies relying on a stock OS (Android) from a company (Google) that's focused on machine learning can effectively compete with the Apple Watch.
The author owns stock shares of Apple.