Other devices (e.g. Galaxy Nexus, Droid Bionic, HTC One X+) married Qualcomm LTE radios with non-Qualcomm SoCs. Even Samsung did this for a few devices (e.g. Galaxy Note and Note 2 LTE, a few S2 and S3 variants), but, until this year, that was a small and shrinking portion of their lineup.
Qualcomm's past engineering success was a big part of the reason why. Samsung/ARM's engineering failures (e.g. big.LITTLE and cache coherency issues with the original Exynos Octa) were the another part of the reason, but I don't think that changes the overall picture.
The Verizon Galaxy Nexus and Droid Charge (both made by Samsung) actually didn't use any Qualcomm chips. Samsung baseband for LTE and VIA Telecom for 3GPP2 (1XRTT/EVDO). http://www.anandtech.com/show/4465/samsung-droid-charge-revi...