I was a Service Unit director 20 years ago -- and I could have written your post then. So trust me, you have my sympathies!
I'm making some assumptions that things are still run today much as they were back then, so please correct me if I'm wrong about any of this.
For those not familiar with leadership roles in Girl Scouts, these are the problems that the OP is trying to solve:
1.) The leader meetings aren't just to provide information to troops. They are also for planning and for recruitment of people to work on events, both of which are much more effective face-to-face. If hardly anyone attends, then you have the same very small group of volunteers (usually no more than 4 or 5) doing all the work. Everyone loves the events, but few want to help make them happen.
Service Unit leaders are volunteers too, and they also have jobs and other commitments (their own troops, PTA, band, sports, church, etc.) Without help, they can become overwhelmed and burned out.
2.) Face-to-face meetings are important so that the SU leaders can answer questions and resolve issues all at once. If everything is handled by email or phone, then the SU leaders may have to answer the same questions many times -- again, this can be quite a lot of extra work.
3.) The SU director can't always refer parents back to the troop leaders. The director is the one who "hires" (and "fires") leaders, and serves as their supervisor. If parents are unhappy with a leader, then it's part of the SU director's job to help solve the problem. (Or it was, when I had the job -- may be different now.)
I wish I had some "solve-everything" solutions for you, but here are a few ideas that might help at least a little.
When you first recruit leaders, make sure you tell them upfront how many meetings are required, and that there must be alternate representation (another parent in the troop) if leaders cannot attend. Let them know from the start that representation at the meetings is a non-negotiable part of the job.
If your Service Unit sponsors any high-attendance events, require a representative from the troop to attend a particular meeting, if they wish to participate in the event. (For us, these were our two annual unit camp-outs.) At least, you'll have good attendance at a couple important meetings.
For older troops (Cadettes and Seniors), the girls themselves might be able to represent their troops at meetings. By these ages, the girls really should have very active roles in planning anyway.
I wish I'd thought of and tried this one years ago -- borrow an idea from Cub Scouts and, a couple times a year, hold a "pack meeting," with a short adult meeting on the agenda. Have an easy activity for families to participate in -- hobo stew, potluck desserts, each troop teaches a song, campcraft workshops. One advantage is that you might get some parents to attend, in addition to leaders.
In spite of the difficulties, I loved my time as a GS volunteer. I wish you every success with your unit.