The "ramp up to full brightness" is exactly what I dislike about the incandescent bulbs. Lights that are meant to grab another driver's attention (turn signals and brake lights in particular) should light up bright and quick. It always amazes me how cars using LED brake and turn lights stand out in a line of traffic.
Plus, I'll bet I'll save a nickel or so in electricity over the life of the car...
As for why some manufacturers stick with incandescent, I would guess that it comes down to cost. At first you only saw LED lights on high end vehicles and then as costs lowered they started filtering down to more mid-level vehicles. But you still don't see them much on mid or entry level vehicles unless they have been optioned with an upscale trim package. Saving a few bucks per vehicle is still a big deal for companies who mass produce vehicles.
2014 BMW i3 with Range Extender Engine, Capparis White/Frozen Black, 20" wheels,, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, TurboCord EVSE