One thing to keep in mind is that satellite design takes into account, in no trivial amount, the specific ship that takes it to orbit. It's therefore, not a simple switch to use a new ship for existing hardware. Everything from G-force limits to fairing payload geometry and weight characteristics and more affect satellite design.

Combine that with the long iteration development of the average satellite maker, it's not always economical to switch to an alternative launch provider, even if the new provider offers a ride for less cost. This is an addition to the reasons such as long term partnerships and discounts.