I suspect it has more to do with the aims and makeups of the teams. TRS, with the exception of Brajkovic, is comprised of old riders. As such, it is likely that Bruyneel would try to burden Phinney with high expectations. And, it is unclear if Phinney would have been allowed to continue his successful track career. Another thing, with an already strong Classic team, BMC would be unlikely to risk Taylor in any of the high-stress and dangerous spring races.
Read between the lines in these passages:
“I'm looking forward to getting into pro racing, riding the front and doing my duties while seeing if I can win some races... I don't want to be the high-maintenance neo-pro. That never works out well. If I get a good base and foundation and a lot of race days, the wins will come naturally[...]"
and
“BMC also makes fast bikes[.] I'm excited to see what they can make for me on the track."
In otherwords, continue to rack up track wins while learning the nuts and bolts of stage racing - learn the Pro-Peloton in a competitive but non-stressful way.