This is a great question and it can be addressed with the following two points:
1. Scope of practice
2. Opportunity
HIS dispense hearing aids almost absolutely. Audiologists can dispense, but they also do MANY other things. It's like asking why the pastry chef makes more desserts than an executive chef. The pastry chef is great at making desserts, but the executive chef can do that and more.
Don't get me wrong, hearing instrument specialists can be very knowledgeable and can serve their patients well, but dispensing hearing aids are their focus, and selling hearing aids is where the majority of the revenue is earned in hearing healthcare, whether you're an HIS or an AuD.
An audiologist, on the other hand, can work in many other important positions that pay less or involve no sales or commission-based payment structure such as in public school, industrial audiology, ENT offices doing just diagnostics, manufacturing as customer support, balance testing, electro-physiology testing, an many other choices that do not generate as much revenue as pure dispensing.
As an audiologist, you will always have plenty of opportunity to move in different directions professionally without having to go back to school. HIS will basically be relegated to doing one function: dispensing hearing aids.
It is up to you to decide what you want to do and how much control you want over your career. If it's just about making money, then maybe the HIS is best for you. If you want to be able to help your patients using a wider variety of solutions AND make money, the AuD may be more inline with what you want.
I say, take the HIS position and see how you like it. Make your decisions based on your experience and continue to ask questions. But I wouldn't recommend pulling your acceptance until you've had time to scope it out a little more.
Hope that helps!