It may just be that I came to barbells from kettlebells and there is enough similarity between the swing hip hinge and the deadlift and that had already strengthened my back.
That's Not It
Yes, there is some similarity between the Kettlebell Swing and Deadlift.
Kettlebell Swings are an effective Auxiliary Exercise for the Deadlift. As I have noted in other post, Heavy Kettlebell Swings (with a Kettlebell that is around your body weight or greater) has been shown (Research Dr Bret Contreras) to produce Power Output that is close to the Olympic Movements.
If (as you state) strengthening your lower back was the key to multiple Deadlift Training Session each week, strong Deadlifters would incorporate Multiple Deadlift Training Sessions. However, the inverse is true.
The majority of good and great Deadlifters only train the Deadlift once a week or every other week. Research by Dr Tom McLaughlin determined what Powerlifter knew for a long time, the lower back is quickly and easily overtrained.
Louie Simmons made a profound statement about the Deadlift years ago, "It (the Deadlift) takes more than it give back." That because of the recovery time required for the lower back as well as the strain on the Central Nervous System (think of it as the "Motherboard").
A More Likely Reasons
Based on a cursory reading of your response, it appears that you are a novice to the Deadlift.
Anything and everything initially works for Novice Lifters due to...
Training Age
Your Training Age refers to how long you have been training, not your chronological age. Research and empirical data have demonstrated that Novice Lifters are much slower to adapt to training. That means they are capable of Multiple Training Sessions with the same exercise each week and can train that way for long periods; around 8 - 12 week before adaptation occurs and they need to make changes.
Advance Lifter adapt much faster and need to make training changes more often; around 3 - 4 weeks.
This has to do with...
The General Adaptation Syndrome
This simply means the body will adapt to any new stimulus at some point or die.
I also lift narrow sumo - just can't get conventional to feel natural at all.
Tip: Do the Squat-Stance Deadlift | T Nation
Your "Narrow Sumo" is a Squat Stance Deadlift. Ed Coan uses this method.
The bottom line is do what feels natural and works for you.
With that said, at time performing Conventional and Sumo Deadlifts will increase your Squat Sumo Deadlift.
Kenny Croxdale