from the school of 'of course we praise our current drivers', last year:
Paddy Lowe, chief technical officer, added: “We have conducted a rigorous and exhaustive driver evaluation process. Ultimately, Sergey Sirotkin impressed the team with his driving pace and talent, technical feedback and work ethic, both at the factory and trackside in Abu Dhabi.
So what changed in a year? $$$ changed.
That's possibly a factor. But also, Kubica had more time in the car, which allowed him more time to discover how the Pirelli tyres work, and how to get the best out of them.
Doesn't fit your narrative though, does it?