Because, you see: 〜もの・〜もん

At the end of a sentence, 〜もの or 〜もん gives a personal reason in an emotional, sometimes sulky tone: “but I didn't know!”. It's very casual, often childish, and often preceded by だって. Its version 〜ものだから / 〜ものですから, on the other hand, is used to apologise politely: “it's just that…”.

• だって、知らなかったんだもん。だって、しらなかったんだもん。But I didn't know!
• まだ子どもだもの、しかたないよ。まだこどもだもの、しかたないよ。He's still a child, come on, it can't be helped.
• 遅れてすみません。道が混んでいたものですから。おくれてすみません。みちがこんでいたものですから。Sorry I'm late. It's just that the roads were busy.

Here, もの doesn't mean “thing”: placed at the end of a sentence, it presents a reason as something obvious you shouldn't have to explain. “But I'm telling you I didn't know!” Hence the sulky, almost childish tone, especially with もん.

• 行きたくないもん。いきたくないもん。I don't want to go!

Use the plain form, often with んだ in front (〜んだもん). After a noun or a な-adjective, keep だ (子どもだもん). もん is more casual than もの.

• これ、高かったんだもん。大事に使ってね。これ、たかかったんだもん。だいじにつかってね。This cost me a lot, you know! Take good care of it.

With だから or ですから, もの becomes the polite way of explaining yourself. You give a reason beyond your control, modestly: “it's just that…”. It's very common when apologising for being late or for a mistake, even at work.

• 遅れてすみません。道が混んでいたものですから。おくれてすみません。みちがこんでいたものですから。Sorry I'm late. It's just that the roads were busy.
• 急いでいたもので、メールを見ていませんでした。いそいでいたもので、めーるをみていませんでした。I was in a hurry, which is why I hadn't seen your email.

から gives an objective reason; ので presents it gently and politely. もの / もん adds personal emotion. ものですから, finally, is an apology: you acknowledge the problem and explain it. Don't confuse it with ものか (next point), which expresses refusal.

• 雨なので、今日は行きません。あめなので、きょうはいきません。Since it's raining, I won't go today. (neutral, polite)