because
the reason is
Structure
- Verb + ものですから/もので、 + Phrase
- いAdj + ものですから/もので、 + Phrase
- Noun・な + ものですから/もので、 + Phrase
- なAdj・な + ものですから/もので、 + Phrase
Informal → もんだから/もんで
Used when giving an excuse/explaining oneself/seeking understanding. If ものだから/もので is on the end of sentence, it means that part after it has been omitted.
Does this differ from ものだから?
Just in formality/politeness. The both carry the exact same meaning. Cheers!
育児は初めてなものだから、いろいろと教えてください
This is the correct answer I got from bunpro. When I gave ものだから it said “you are missing something”, and when I tried のものだから it marked it as wrong.
But I don’t get - what is the な doing here? Why does it seem here like 初めて is acting like a na adjective? and is の really incorrect here?
Funny we got the same review on the same day.
The way I understood it after double-checking the “structure” block, is that nouns connect with な in this case, not の as is more typical.
Noun + な + ものですから (1) + Phrase
Oh, I didn’t think 初めて could be a noun, I thought it is just the て form of 初める and couldn’t understand why a te-form verb would take な, but I know see it is marked in jisho as noun, so that makes sense 
Thank you!