For a) by their subservient and fearful fellow soldiers; for c) by anyone with a grudge including ISIS.
To switch sides would take a large number of them who can confidently discuss things free of threat from those who are strongly committed to Assad and there would be a great many who feel their somewhat comfortable existence is too much to risk with joining a revolt.
And IS isn't the only alternative. When the civil war started IS wasn't involved and the Sunni Islamists were on the fringes of things. IS may have become one of the better funded 'rebel' groups but they are far from the only ones.
There is no alternative to ISIS. The remaining groups are just as bad. What was the US funded 'Free Syrian Army' does not really exist anymore, as their members have either been wiped out, sold their weapons to ISIS and run, or reportedly most have joined ISIS. Maybe ISIS disn't exist when the US/Saudi funded insurgency started, but they do now and the original situation is gone forever. There are few so-called moderate rebels left; the most recently US trained ones were ambushed almost as soon as they set foot in Syria. The other ones, like the 'moderate' heart-eating Jabhat Al-Nusra Front are just as evil, although some US neo-cons and the Israelis were recently trying to make us believe that they are 'moderate' so as to be seen as still supporting an opposition to Assad for his overthrow. Israel is providing support to them, because the primary object is to overthrow Assad, and accept a territory next door where chaos reigns at the hand of terrorists.
The ones committed to Assad are members of his own group, the Alawites, who represent the majority of the officer corps, but only 14% of the population. However, the officer corps has a lot of Christians and Sunnis also, and 76% of the army is still Sunni, and it's very easy to drive your tank and your 2 other 'Sunni' crewmen (in a T-72) away towards enemy lines; the army is intermixed, and the sunnis are the majority in most units. You have no idea of the existential threat ISIS poses to secular Syria. The fact that nobody in the army wants to defect to ISIS is because of the consequences, and that ISIS is now squeezing Assad's Syria into a quarter of what it used to be, means that there is nowhere to go except fighting for your homeland or flee to refugee camps in neighbouring countries or Europe.
Politically, the 'Moderate Syrian Opposition" and Syrian government in exile was a greater joke. It was composed of US paid-off exiled Syrians, some of whom had been away living in the US for at least a couple of decades, and some of whom who were, curiously enough, living very near Langley VA (CIA HQ) or within spitting distance of Capitol Hill. Their coalition of dissidents was never able to achieve a consensus, and all cohesiveness fell apart in 2012.
There was a local Syrian Sunni opposition, and they had been working with Assad for better representation, but they got eclipsed by the US funded exiles, and then the civil war funded by the Saudi Arabia, Qatar and the US.
An example of how little support the US funded Moderate Syrian Opposition had amongst Syrians, is when one of their representatives came to Ottawa to explain their position and try to whip up support. The meeting (I think it was in a Carleton University building) had barely started and there were maybe a couple of dozen people in attendance, when a group of middle-aged or older ladies with handbags started shouting 'Get him! the Judas! Kill him!' and this gaggle of old ladies chased him out of the building and onto the parking lot. There was also a pro-Assad march in Ottawa last year, from Elgin Ave to Sussex Drive in front of the US Embassy with about 200 Sysrians and Lebanese marching. A large portrait of Assad was carried foremost, and pro-Assad chants were quite evident. I had a dentist recently who is Syrian, tell me how wrong it was for Assad to be vilified; Assad is the only one who can get the various groups to live together in Syria, and who can guarantee religious rights to minorities, including the 10% Christina population and the Shiites. The Free Syrian Army had already been desecrating and looting churches and convents and monasteries, and they were US and Saudi funded. Imagine what ISIS is doing. Christian Syrians are openly fearing that their fate will be a repetition of the mass exile and elimination of Iraqi Christians from Iraq since 2003. There are only 10% left out of total Christian population of over 1.5 million.