The Scotland defender has signed a pre-contract with the Ibrox club but had other offers on the table from clubs in the English Championship.
However, he did not want to leave Scotland and be further away from his very sick relative, according to former Hearts boss Levein.
Souttar has criticised by some fans since the news of the deal broke.
But Levein, who signed him from Dundee United in 2016, told BBC Sportsound: "Very sadly, a family member of John's has a very, very serious illness and he doesn't want to leave Scotland at this minute in time.
"Obviously the only option on the table, other than staying at Hearts, is Rangers.
"I believe that's why, yes he wants to win trophies and to be successful, but his primary concern this minute in time is that he doesn't leave Scotland."
Souttar remains a Hearts players until the summer, unless the club accept an offer from Rangers to take him to Ibrox before the end of the January transfer window.
It remains to be seen whether or not he’ll be involved in Tuesday’s Premiership match against St Johnstone at Tynecastle, but Robbie Neilson is due to face the media tomorrow and can expect to be grilled about his Souttar predicament.
The Hearts boss told the Scarves Around the Funnel podcast last week: “We have to keep our best players in the team. If that means it’s at the end of the season they go, then it’s the end of the season when they go.”