not only...but also ...

"not only" is one of a number of words or phrases that, when placed at the start of a sentence, trigger the inversion of the subject and verb that follow. Other examples:

Little did they know.
No sooner had we arrived than it was time to leave again.
Rarely have I been so angry.

I want to know the reason of it?

Form1: Some of these pigeons would have arrived by now if it had no been for the thunderstorm that cut them off for two days.
Form2: Some of these pigeons would haved arrived by now had it not been for the thunderstorm that cut them off for two days.

What is the aim of the reduction from Form1 to Form2?

w415384914I want to know the reason of it?

There is no rational reason. It is just the way it is.
Subject verb inversion is used in a number of cases.

w415384914I want to know the reason of it?

I don't know. I don't think the reason is clear to ordinary modern speakers. Possibly one would have to look back at the history of the development of these expressions, which would be a very specialist area.

Any ideas, anyone?

Form1: Some of these pigeons would have arrived by now if it had not been for the thunderstorm that cut them off for two days.
Form2: Some of these pigeons would have arrived by now had it not been for the thunderstorm that cut them off for two days.

These are just two different ways to express a conditional. Here is a related example:

Had it not been for the

GPYAny ideas, anyone?

The Germanic languages, from which English was derived, have a different word order than modern English (V2 word order versus SVO). The auxiliary verb inversions in Modern English are the V2 order, and a vestige from Old English, which was much closer to the Germanic roots.

I think the reason is for simplicity. Because the if-condition expression is a little long. In order to reduce its length, we use
the inversion structure.

w415384914I think the reason is for simplicity. Because the if-condition expression is a little long. In order to reduce its length, we use the inversion structure.

That's unlikely.

why