It is no surprise that the Caps won in '18 when they were able to ice two very good lines vs. one loaded line. Kuzy playing well allowed the Caps to finally separate OV and Backstrom; their two best offensive forwards.
The Pens were always able to separate their two best forwards because Crosby could carry a line of lower depth chart forwards and produce world class numbers (in '09) or effectively play a 2-way C role (in '16).
Ovechkin could never carry a line on his own to a point where the Caps would load up a 2nd line. He lacked the offensive versatility.
It is also no surprise that when OV clearly played a more responsible defensive game in '18, the Caps won.
