The leading faction, Hayat Tahrir al-Sham, is in part a descendant of the Islamic State of Iraq, as ISIS was known prior to its direct engagement in the Syrian war. The current HTS leader Abu Muhammad al-Julani was initially sent to Syria by Abu Bakr al-Baghdadi himself, so the links to ISIS cannot be denied.
That said, as al-Julani’s group have evolved through several iterations in the context of the Syrian war, and incorporated a broader coalition of opposition groups under its wing, they have developed a more populist approach to combat and governance in Idlib, where they have held sway for a long time. Al-Julani has sought to avoid the type of alienating extremism that ultimately turned so many against ISIS, and has demonstrated a willingness to work alongside a variety of alternative opposition groups, some of whom collectively fall under the rubric “Free Syrian Army.” They have not engaged in international attacks or the most brutal type of violence associated with ISIS. In adopting this approach, they have acquired some mainstream, popular appeal and credibility (again, in the context of this war), and therefore are not to be dismissed merely as Wahhabi fanatics or whatever.
And of course, it should always be remembered that in the context of this horrific war there are a multitude of possible factors driving recruitment for a group such as HTS, not least of which remain the nature and actions of the Assad regime itself.