In Fantastic Beasts and Where to Find Them, we discovered that any witch or wizard with their magical powers repressed eventually turns into an "Obscurial". In the Half Blood Prince, we learned about how badly Merope Gaunt was treated by her brother and her father. She knew only rudimentary magic and ran away with a muggle when she got the chance.
Merope never suppressed her magical abilities
Obscuruses develop under highly specific circumstances, which don't appear to be satisfied by Merope's situation:
Newt: Before wizards went underground, when we were still being hunted by Muggles, young wizards and witches sometimes tried to suppress their magic to avoid persecution. Instead of learning to harness or to control their powers, they developed what was called an Obscurus.
Fantastic Beasts and Where to Find Them (2016)
An Obscurus is the repressed energy of a child who is forced to hide his or her magical talent. This energy can manifest itself as an entity on its own that can erupt in violent, devastating fury.
Fantastic Beasts and Where to Find Them: Magical Movie Handbook "Beasts" The Obscurus
An Obscurus is developed under very specific conditions: trauma associated with the use of magic, internalized hatred of one’s own magic and a conscious attempt to suppress it.
While Merope unquestionably suffered terrible abuse, and that abuse was related to her magical ability, she was never directed to supress it, or taught to hate it, the way Creedence Barebones was; quite the opposite, in fact (emphasis mine):
"Pick it up!" Gaunt bellowed at her. "That's it, grub on the floor like some filthy Muggle, what's your wand for, you useless sack of muck?"
"Lucky the nice man from the Ministry’s here, isn't it? Perhaps he'll take you off my hands, perhaps he doesn't mind dirty Squibs...."
"Is it true?" said Gaunt in a deadly voice, advancing a step or two toward the terrified girl. "My daughter — pure-blooded descendant of Salazar Slytherin — hankering after a filthy, dirt-veined Muggle?"
Half-Blood Prince Chapter 10: "The House of Gaunt"