Here are some possible explanations:
That “noon” is called noon alwiqayah (noon of protection or protection noon). it is used to preserve the vocalic environment of the conjugated verb when it is added to the objective particle ـي (a vowel).
And because verbs already end with vowels (declensions), adding another vowel will result in a mutation of sound. which will change the meaning.
If I want to say “I marked down myself” I would say :
katabat-u + y, but the ending sound -uy isn’t a common sound and it risks to become katabaty (you (anti) wrote), which will destroy the conjugation and mix them.
Without the ن it would be سألتي
Which when spoken sounds similar to (fem. 2nd pers.) أنت سألت anti sa’alti , therefore the ن is added to avoid the confusion between them
Attention, the following might seem complex or far fetched:
Why the ن was chosen and not any other letter? Because every personal pronoun has a letter to represent it for substitution that comes from possessive pronouns, for انا it’s ي – for انت it’s ت – .. etc
The ي of انا cannot be used in this particular case, so the ن of نحن = we, was used instead.