TLDR version - expensive to get someone that might only be a short term employment
In order for a company to sponsor a non-citizen they have to show that there is a lack of citizens that have the correct skill set to fill the job openings. This is a long drawn out process where they have to establish the need, show that they have tried to get people for the position and that there is no one able to take the job for a certain length of time. This might mean they have to be seeking someone for 9 months and show that no one has been qualified for the position in that length of time. If they then want to sponsor someone they then have to find the person, get a work visa (lottery system with only limited numbers per year) do the paperwork, be able to get them a work permit, pay the fees and then have the person come over to work for them. Most times this involves signing longer term contracts since they have a large investment in the person at that point and the last thing they want is for that person to come over and then leave 2 months in since they don't like it anymore. Most DZ's do not hire employees but instead hire independent contractors - aka freelancers that are not on a contract and can quit/leave at any time and have a lot of other things for taxes that the DZ's do not have to cover.
With all this being said a DZ might have to spend 5-10+ thousand dollars to sponsor someone to come from overseas or they can just find a local for free. With most DZ's that need staff are in a seasonal location that means they might be paying a lot of money for someone that might only last 5-7 months before the DZ shuts down for the winter and then that person decides to go somewhere warmer and then will have to get that person to return to them in the spring. If the sponsored person decides not to go back or leaves the country then its potential legal bills to sue over the situation or writing off the costs of it. It has been done for the right people - high level coaches, specialists like videographers with movie level experience and others that have skills and personal relationships with DZ's that make the investment worth it for them. When a Tandem Instructor only makes say 40k USD in a year paying out another 10K for a non resident alien to come over to get the job is usually not an investment that DZ's are willing to make unless its for someone with extraordinary skills or abilities that has the potential to recover those costs and make the DZ more money. Think of a high level coach - DZ's could make their money back and it promotes the DZ with all the teams that the coach will bring to their DZ that they would not have had otherwise.
Tandem instructors and AFF instructors are just not lacking enough for the most part to show the need to the US government that they need to allow lots of nonresidents to come over to fill the need.