Hi Marius,
I believe it’s a combination of things:
1. TSMC were one of the first independent foundries in the world, and they were back up by the government at the beginning.
2. Quality.
3. Professionalism.
4. Responsiveness both in the fabs and in the sales and support offices.
5. Delivery on time or even before.
6. Broad range of CMOS processes.