All Praise be to Allah (swt),
I think that there are a number of factors that are at play, one being the lack of knowledge. This is why gaining religious knowledge is obligatory especially in matters relating to our aqeedah and authentic creed. Someone would say to me, "Really?" I would reply, "Yes, really". They would say "I believe in Allah (swt), the Prophet (pbuh), am a good person and don't hurt others - that's all I need to believe". I would reply "Noooooo... How and when do you we know that we are not doing something that is completely negating our aqeedah?... Yes, there is a multitude of factors that take us out of the fold of Islam while claiming to be muslim and we can protect ourselves from them by gaining knowledge!"
To make it short. Knowledge brings the love and fear of Allah (swt) and the importance of loving and following the Prophet (pbuh)'s sunnah according to the understanding of the righteous predecessors. Once a sister has taqwa of Allah (swt), she becomes mindful that Allah (swt) is watching her every move, every thought, every turn, every look, every intention, every everything. If she prays for guidance and if Allah (swt) wills, she will get guidance and with it will come the love to obey the orders of Allah (swt) including the wearing of the jalbab/hijah/niqab properly.
If a sister doesn't wear or properly wear hijab, she may not have the complete knowledge or proper understanding of it. For example, every single time non mahram men look at her accidentally, intentionally, lustfully enjoying every look, she gains a sin for every single gaze by those men. Imagine, walking down the street, men looking at a sister up and down, front and behind, she cannot comprehend the thousands of sins being added to her book of deeds. Inshallahu tala, if anyone is interested, I can provide textual evidence from the Quran and authentic Hadiths.
May Allah (swt) give us all guidance and protect us from the whispers of the Shayateen. Ameen