Why does the Boeing 777X have a flight deck similar to the 787 and not to the previous 777?
The reality is that the 777X is not all that different from the 777 when considering cockpit procedures or 'user interface'. What is different is the hardware platform(s) that are used.  

I worked for a supplier to Boeing for the 787. The stated goals for the 787 from Boeing were fairly straightforward. The design shall require a maximum of 5 days transition training for 777 pilot with a goal of 1 day. On the support side, there was a push to greatly reduce maintenance and upgrade costs. These are driven by the hardware and system architecture.  

So the big change with the 787 was moving from the Honeywell AIMS and A629 based network to the Collins/GE A651 and A653 based Common Core System and A664 based network. The result of that platform change is why the flight decks look so different and that's what you see in the videos.  

I can say with authority that in the 787 design our overriding directive was to make it look and behave like the 777. While a keyboard and a window on a widescreen display looks different from a control and display unit (CDU), the key functions and display behave the same.  

So now with the 777X, the hardware is an evolution of what's in the 787. It uses the same basic open architecture and also follows the tenet of minimal changes that will require crew training.  

So ultimately, the 777X is operationally similar to both the 777 and 787 while the hardware and software infrastructure follows the 787.  

The impact of training is significant.  As explained to me by a United rep, when one senior captain retires it generates 10 to 15 training requirements as crews move up in the system. Every day of training costs them a lot of money.