Until CDMA goes away, Samsung will struggle to remove carriers out of the equation. Qualcomm gets to have a stronghold on the American market. Other OEMs should abide by Google's standards before things get worse and worse. HTC used to be the standard for third-party Android OEM updates.
It has nothing to do with Qualcomm. Qualcomm has ZERO impact on deploying monthly security patches (only full OS updates), this lies 100% on the device OEM and the carrier's desire to add its own twist on the configuration in the form of apps added. Where it doesn't make any sense is that the unlocked versions of devices that also have carrier-branded device in the US seem to take longer to come out.
Take my Tab S3 for example - this device is not running a Qualcomm SoC, yet it took SIX MONTHS between deployment of "monthly" security patches. Until March it was stuck on the August, 2017, patch. This means this $600 tablet was left vulnerable to KRACK for 5 months. That's just plain unacceptable.
The issue with a company like Samsung is one of workload. Look at how many different devices Samsung releases in a year - they need employees on the testing teams to test patches against every one of those before release (not to mention also to develop the update), I imagine that stretches the teams quite thin.