The guy who is the public face of HTS headed up the Nursah Front or whatever for a while before HTS, and they were allied with Al Qaeda for years (they split, but then he has been working with them since as the HTS head), so yeah, he was affiliated with Al Qaeda - not good, Bob, not good.
On paper the Syrians should have defeated them (they should have outnumbered them well over 10:1), but there were a shitload of Syrian defections either to what became HTS or to their own militia groups or they just quit and went home, and Assad apparently couldn't deploy certain troops to certain areas because of loyalty issues. Hell, Syria has several thousand armored vehicles and an Air Force, but still couldn't defend their cities when the time came.
Okay, now look at Iraq - on paper they have close to 180,000 active duty troops, which means *probably* around 30,000 actual combat troops/infantry, give or take a few thousand. Iraq is a big country and those troops are spread out, and plenty are going to be on the Iranian border trying to stop weapons/personnel going through there to Syria and elsewhere and plenty are going to be on the Syrian border for the same reasons (but then a bunch of Syrian troops defected to Iraq in the last week or two).
They (HTS) are very good at exploiting divisions between various religious groups and/or bringing together disparate groups and recruiting local groups, and they are very good at mobile warfare. HST spent a few years really training and transforming how they fight, and I doubt the Iraqis are on their level, or at the least aren't nearly as motivated and mobile.
These things happen slowly until they don't. If they move on Western Iraq unopposed outside of Iraq, and take it quickly, there are going to be Iraqis and others from outside of Iraq who are drawn to it, who are going to support them. Once they get setup in Iraq, they will pull in plenty of local Iraqi groups who have a beef with the Iraqi government, Iraqi Shiites, etc.
What happens if they take Baghdad? All of the sudden, they arguably have their caliphate, that just so happens to be sitting on the border of Saudi Arabia/Kuwait, and stretches from Lebanon/Mediterranean to the Persian Gulf.
That's scary for the Middle East and for the West. The only good news is if they want to rule/occupy the areas they've taken, they have to tie at least some of their troops down to fixed locations.