Some TT bikes can still be above the UCI min, if only just.
There is also a lot of perception that road bikes go uphill faster.
For road stages, there can be other reasons. IIRC at one of the Tours (or perhaps the Giro?) in the last couple years, Contador would switch bikes either before or just after the climb, with the speculation being that there were different tires on one of the bikes to better cope with crappy road conditions going downhill. Of course, others speculated it was a switch to an uninspected or underweight bike.
You can read about it here.
There is also a lot of perception that road bikes go uphill faster.
For road stages, there can be other reasons. IIRC at one of the Tours (or perhaps the Giro?) in the last couple years, Contador would switch bikes either before or just after the climb, with the speculation being that there were different tires on one of the bikes to better cope with crappy road conditions going downhill. Of course, others speculated it was a switch to an uninspected or underweight bike.
You can read about it here.