I may have yet to say this, but I love the way you design machines and some of you other characters as well. Some of them have such a nice, elegant and slender design and they seem so graceful. You have a lot of interesting idea when you come up with a concept like this and its always fun to see the final result.
Well given that ALL ponies have magic. (earth magic, air magic, "magic" for unicorns). You could translate it as programming and software
The others have programs and software which focus on their physical selves and functionality, running their machine bodies and carrying out life-purpose functions. For earth mecha ponies its physical strength, load bearing etc etc. With Pegasus Mecha its flight control and weather control, etc etc.
Unicorns are closer to their internal coding, programs and the nature of software creation and editing. They directly use it to control the world, usually in the form of levitation, bending of physical reality, etc with focus on an intended life purpose (cutie mark, their core program). Twilight is life purpose itself focused solely on this "magic", software and programming, and thus exists only as software. Her only presence in the world being her "visual representation" as a holographic avatar.
Seeing as how Twilight is the only AI in the group who doesn't have a body (unless you count her projectors)... think she'd feel like she's missing out on something, since all her friends are physical?