OK maybe was a bit harsh but the word 'justify' was used and that struck a chord.
To summarise reasons mods are not included -
1 Like the mod but it was not ready/stable at the time of putting mod pack together
2 Hate the mod - would never use it
3 The mod is fine but there is another mod or a group of mods I am including that do the same things so I don't need it
4 The mod does not fit the theme I am going for
But if what you actually want to know is not 'why the mod was not included' try the direct approach - if you really want to know if the reason the mod was not included is due to compatibility issues then simply ask "are there any known compatibility issues with" mod x and mod pack y.
As well as not getting reply 2, 3 or 4 above ,if the author answers, which does not answer what you really want to know, it gives you the opportunity of getting a response from someone who has tried to add it, if they had problems and how they solved them.