Hello,
I have installed 4 new Kumho Ecsta Platinum LX tires 2500 miles ago. Replacing the Goodyear Eagle RS the car came with.
Anyway, I'm going to rotate them at the 5k miles mark (owner's manual says 7.5k). I had the same tires on my previous Maxima 2002 and always rotated them in X pattern (FL to RR and RL to FR).
Why owner's manual states to rotate front and rear on same side only? Could it be because of the stock tires?

I believe it is to keep the tire spin direction the same. The X pattern reverses the spin direction.

In the old days Radial tires needed to be rotated front to back, otherwise they odd shift in the steel belts could be bad once the rotation went the other way.

Then you also have directional Tires and Rims. You can see when someone has directional rims and there was a X rotation done, on one side the wheels look to be going in two different directions, the other is the tread may require a specific direction, if you look at the sidewall there would be an Arrow depicting the direction the tire must rotate.

Now if you are not using radials or directional rims or tires, you could do the X pattern. But in my honest assessment, I don't think it makes the tires last any longer vs the front to rear. Bottom line as long as you are rotating from drive wheels to non drive wheels, and from front heavy to rear lighter, you should get the most out of a set of 4.

That said if you had staggered wheels, you're going to never rotate tires in most cases. You could go right to left, but chances are with staggered rims you'd have quality directional tires...so you will be replacing one set, front or back, more often then the other. Depends on drive wheels and weight distribution and driving style as to which set will wear out first.

As to why Nissan says to do front to back, the probably figure due to so many variables with rims and tires, directional and non, that it's just safer to tell people to do it only this one way, front to back.

As you described you were doing X pattern and had no issues. So as long as the tires aren't directional you could do it either way, just be sure fronts get to the back with each rotation,

I always rotate mine front to back and back to front as stated in the manual, Haven't had any issues and honestly like MadMax said above as long as the wheels are getting their fair share of use every rotation is all that matters.

I run winter & summer tires. I always put the better tires (deeper tread) on the rear of the car to avoid the rear kicking out and loosing control.