Motivation is definitely one of the most important factors. Firms know how tough it is for trainees, that you're going to be working long hours under a lot of pressure. They need someone who will thrive in that environment and work well with the rest of the team. It's why they want someone who has a genuine interest in commercial law and a genuine interest in their firm. It's an expensive investment on their part and they need to believe you're right for them.
This is why research and knowledge can only go so far. If you've met the firm before, this will go down well because you show you've made the effort to meet the firm. It's also a lot about the way you come across in an interview. Recruiters are pretty adept at picking out which candidates have a genuine interest, compared to the candidates who recycle deals they've read or tier one rankings from Legal 500. When you're answering "Why our firm?", try to go beyond a rehearsed, surface-level answer. Really think about why you find their training contract structure or their international reach interesting and why you'd choose them over any other firm. These things can be tricky to find, but again, you can get better with practice.