Soft skills are as important, if not more important, than technical skills

Unfortunately, his career progress was sluggish. It was hampered by the fact that it was not easy for others to work with him due to his difficult personality and meager soft skills.

Skill #1: Marketing your work
The impression that managers have on us is based on their observations. They piece together an image of your performance based on contact points like:

your direct communication with them (1-on-1 meetings, working together on projects)
meetings they attend where you are present
demos and presentations that you give
your communication with others where they are passively involved. For example, email CC.
what others are saying about you behind your back (yes, people do talk behind your back)

Skill #2: Time management
Skill #3: Networking
Skill #4: Open-mindedness