It's a combination of several factors, for example:
1) For some the sim doesn't work to an acceptable standard, even though it's leaps and bounds ahead of what it was initially and most of the people are actually flying by this point in time.
2) For some the sim is a bit complicated and they are not being in their comfort zone just yet. The amount of people asking how to turn on their engines or why the gauge needles shake in the ATAG server chat are a testament to that and i've only been online twice, imagine how many times the regulars have seen it pop up
Which brings us to the final point...
3) Official documentation is good and well laid-out in terms of how understandable it is, but doesn't cover a lot of things. Meanwhile, people either tend to be a bit "lazy" at times due to real-life getting in the way of their available flying time (and who can blame a young father who only has a couple of hours tops to spend on the sim between work/home/kids/etc), or just expect to jump right in and do reasonably well because that was their previous flight sim experience, so they often don't bother to read up on the community resources we got stickied here
Long story short, take the time to type a couple of lines in chat for the rookies telling them to visit here and check up on the sticky how-to threads: saves you time trying to explain everything mid-session, saves them time getting up and running. Maybe it would even be a good idea to have server transmitted messages that point the newcomers to community resources.
I think that even when the sim is fully, completely optimized and documentation and community resources are better, the complexity will still be a factor that slows down the rate of online participation even for those that do like to fly full switch and welcome the challenge. I mean, wanting to fly with CEM on didn't automatically make me a guru on internal combustion engines, there was a lot of trial and error involved and quite a bit of reading time over some months to help me get to a simply acceptable standard (that was before CoD though, so i was more or less ready when i got it).
I'm not trying to slight anyone mind you, to the contrary in fact: I find it absolutely natural and in agreement to my own previous experiences that it takes a reasonable amount of time before everyone is up to speed with the new CEM and comfortable to fly online on a full switch server.
In my case i learned what i needed to a couple of years ago out of my own curiosity, thanks to flying some high quality add-on FSX aircraft on a friend's PC whenever i would visit him over the course of a couple of months.
If i was just now starting to know CEM i would be totally swamped and not at all confident to go online, make no mistake about it