Flexi is good for sending artwork to a vinyl cutter/plotter or doing certain things to prep files to send over to EnRoute. As a design tool, Flexi is badly dated -especially with how it handles fonts. It doesn't provide much support for advanced OpenType features, much less support newer formats like OTF Variable fonts. This is a common problem for just about all sign industry specific "CAS" software. The situation is pretty stupid. Applications like Flexi are pretty expensive. Typefaces are pretty important in sign design. So why aren't "CAS" software vendors like SAi on the cutting edge of font technology? When it comes to type these applications are literally stuck in the 1990's.
Even with programs that are industry specific programs along the lines of Flexi (just for different industries) are not only equally as expensive (or I can think of one that is twice as much for the full version), but equally behind the times in terms of functionality when they add design functionality that can be handled with Ai/Draw directly within the program. Not so much the UI, that aforementioned one that I was thinking that is twice as much as Flexi, has updated the UI. In fact, they were so gungho with the UI, they changed it the last two versions in a row (and for me, not necessarily in a positive direction, but that is just me).
As to why, very easy. There really isn't a mainstream competition that most people in the industry would know about to have as an alternative (or there isn't an alternative period) that would force them to change, to innovate unless they are able to buy their innovation. There may be plugins for Ai/Draw that will give some functionality sending to the cutter/printer/engraver etc, but they will still lack some of the more advanced features that dedicated programs have in those areas. It just depends on what ones needs are. But for the most part, a plugin is still dependent on another program as well. Nothing really beats true integration and depending on the plugin system, that may not be doable with the host program (some programs it is, some it depends on how much the plugin vendor pays for source code access etc, but not all).
I have to wonder, while design is important step before getting to this programs, I have to wonder if the design implementation directly in these programs, are more for down and dirty quick design needs versus needing a whole specific suite of tools. Otherwise, development should be more rapid compared to what they are, if they are really trying to implement a full design suite, making their program a one stop program. Which very few programs, truly implement that in such a way that they do it justice. Best to stick with one thing and that one thing well. Sucks for users on one hand, but it also makes the individual experience better on another.