To be fair to Rafa, he came close to winning it a couple times, and went deep four more...two Finals, four Semifinals. He even beat Roger in the 2013 SF. Meaning, he wasn't a pushover at the Tour Finals, he just never won it.
Fun fact: Roger, Rafa, and Novak all lost 18 career matches at the Tour Finals, though with rather different resumes:
Rafa: 11 events, 21-18 (53.8%), 2 finals, 3 semifinals
Novak: 16 events, 51-18 (73.9%), 7 titles, 2 finals, 3 semifinals
Roger: 17 events, 59-18 (76.6%), 6 titles, 4 finals, 6 semifinals
Andy: `8 events, 15-12 (55.6%), 1 title, 3 semifinals
Kind of crazy that in 17 events, Roger didn't make the SF only once - all thanks to losing to Andy Murray and Gilles Simon in 2008.