My question is: in column piping guides used in vertical straight run are at unequal distances. Is there any specific reason behind this?

Usually a uniform spacing is preferred as you imply. Some reasons for unequal spacing:

1. Appurtenances in the way.
2. Part of a Recip compressor system where it is good practice to vary the pipe spans so adjacent spans have different natural frequencies.

In general, for vertical runs, you want different spans between the guides so that, if you hit a harmonic in one of the spans from the vortex shedding of wind, it will be less likely to propagate to other spans as they will have different natural frequencies.