Victor's entire project of creating the monster is one that he toils over mostly in isolation from the very beginning. When he became consumed by the occult science he was studying and the progress he was making in his work, he noted in Chapter 4:
"I knew well, therefore, what would be my father's feelings; but I could not tear my thoughts from my employment, loathsome in itself, but which had taken an irresistible hold of my imagination. I wished, as it were, to procrastinate all that related to my feelings of affection until the great object, which swallowed up every habit of my nature, should be completed."
Since he knew that his father (and likely other friends and family members) would dismiss his work as crazy, obsessive, and dangerous, he hid his work away from them in order to complete it.
Later, once the monster has come to life and begun to exact its revenge after being abandoned by systematically murdering Victor's loved ones, the monster demands that Victor create for him a partner, and then the monster and his wife will seclude themselves never to be seen by mankind again.
To create the monster's bride, Victor chooses a remote spot. In Chapter 19, he explains:
"Having parted from my friend, I determined to visit some remote spot of Scotland, and finish my work in solitude. I did not doubt but that the monster followed me, and would discover himself to me when I should have finished, that he might receive his companion.
With this resolution I traversed the northern highlands, and fixed on one of the remotest of the Orkneys as the scene of my labours. It was a place fitted for such a work, being hardly more than a rock, whose high sides were continually beaten upon by the waves. The soil was barren, scarcely affording pasture for a few miserable cows, and oatmeal for its inhabitants, which consisted of five persons, whose gaunt and scraggy limbs gave tokens of their miserable fare. Vegetables and bread, when they indulged in such luxuries, and even fresh water, was to be procured from the main land, which was about five miles distant."
While there is some evidence at this point that Victor, knowing the monster is following him, is trying to create some distance between it and his family and loved ones (including his best friend Clerval who he was just travelling in Scotland with), but I'd argue that his picking such a remote and secluded spot mirrors the isolation in his early attempts.
Victor knew others would judge him harshly and try to interfere with his work, so his pride and obsession compelled him to keep it secret. Now, he still hasn't admitted to anyone what he's done, and the shame and guilt at the consequences of his actions (the death of his loved ones at the hands of his creation) continue to force him to work in seclusion because of fear of being found out and, therefore, having to truly admit the evil of what he's done.