Because until the C1, Apple did not have their own, and there were not that many viable options available with Qualcomm 5G modems being the «best» (however one defines it) and the fastest albeit power hungry and running very hot at high speeds.

Outside Qualcomm, there has been a limited number of players out there, with MediaTek seeming to pick up the pace and giving Qualcomm a serious run for its money – if not now then pretty soon.

What is left is… not a lot and appears to have constraints of one sort or another:

  Samsung – with 5G-integrated Exynos SoC's. It doesn't make sense for Apple to house 2x SoC's in the same appliance;

  Huawei – they target the mainland Chinese market with its own flavour of 5G. One doesn't want their modem anyway due to national security concerns;

  Sequans Communications – they focus on the IoT market, which is a niche and has its own unique constraint space;

  Intel – they quit the 5G modem market in 2019 and sold the IP to Apple, which has given them the C1/C1X.