The GSEA manual states that the signal2noise ranking metric should be used only in case of 3+ samples per condition.

What is the reason for this restriction?

I do understand that the denominator has standard deviations for the input values and it may be more meaningful in case of 3+ samples but how is it unsuitable in case of 2 samples?
I guess it is partially arbitrary, but the confidence intervals improve a lot with 3 samples, compared to 2. Suppose a SD of 1, then you have the following [confidence intervals][1]:

    N=2		95% CI of the SD 		0.45 to 31.91
    N=3		95% CI of the SD 		0.52 to 6.28
    N=4		95% CI of the SD 		0.57 to 3.73
    N=5		95% CI of the SD 		0.60 to 2.87

  [1]: https://www.graphpad.com/quickcalcs/CISD1.cfm