1) Are the 777-300ER's actually wider than the 772, or they're just cramming in more seats into the same space?
2) But isn't the dreamliner way more attractive to travellers? I mean, just the name....
All the aircraft in the same family generally share the same fuselage width- all 737 variants, A318/319/320/321, the A330/A340, all 777 variants and all 787 variants have the same width.
Airline and their boards purchase aircraft that most cost effectively meets their anticipated network needs and aligns with existing maintenance capabilities and operations. While Boeing has successfully marketed the desirable passenger focused characteristics of the 787 (they also did so on the 777 back in the day) as Airbus has done with the A380, they aren't first and foremost when making strategic planning board level decisions.
Commercial aircraft should be thought of as building a custom home, more so than an automobile. Again while certain aircraft have structural characteristics that make them desirable, an airline can choose from a multitude of seat, IFE, component and other suppliers to customize their interior. Talk to anyone coming off AC 787 TLV-YYZ in the main cabin you likely won't be hearing singing praise for the very dense cabin.