Apple had given Intel modems a shot, and then users started complaining about performance differences. In some cases the Qualcomm modems would outperform the Intel ones by over 70%, especially in situations with low/weak signal. To maintain consistency and avoid outrage they tried to intentionally nerf the Qualcomm modem performance in these situations (since the Intel ones were lagging behind significantly).
Furthermore, the iPhone X was intentionally only made with two antennas, even though the Qualcomm X16 supported 4x4 MIMO and thus gigabit LTE because Intel's modem at the time didn't support that. Once Intel could do it, they released the iPhone Xs which is the same body but with the correct antenna support.
