Indeed interresting discussion which seems to be a recurring one, but
I am surprised to see nobody object to the fact that modx is complex and requires advanced coding skills
modx is neither complex nor does it requires extensive coding skills !
Granted if you need to push a CMS to the limit and exploit its power, you have to be able to code php, but one of the things that attracted me to modx was that you can go further with modx without knowing php than any other CMS, period

As it happens, my php is rudimentary and I have done some interresting stuff without php, things I wouldn’t have dreamed of with any other CMS.
I’d say modx is not
complex but
different, the same way textpattern is different and I recall many very similar discussions during my time there...
Using tags to call snippets is a simple but powerful idea (txp use the same logic with plugins), because it’s way more efficient to learn a simple syntax a refer to a set of parameters than learn complex module admin and find your way back to a setting. Also, it fits naturally in a HTML page, also a tag based syntax though less powerful than modx’s. CMS Made Simple is an interresting case because it has one click features though it also has a tag syntax which is very easy to learn if you come from textpattern and fairly easy for any modxer. But CMS Made Simple suffers from a great lack : there is nothing close to TVs in modx or CCK fields in Drupal (last time I checked at least) and to me custom data structure is one essential key of content management.
Now that I had to work extensively with Drupal I think I have a nice insight at
what makes modx different : I can adjust snippets parameters right in my templates of my chunk, I don’t need to go to the admin, pick the module, go to the settings, adjust, go back to my template (tiring, very tiring). Even the views module for Drupal which has a nice UI and basically has the same purpose Ditto has for modx, I find it much less efficient and it forces me into Drupal’s workflow rather than fit my workflow like modx does (and I find myself using the insertview module extensively, as it brings me back to a tag based syntax to adjust my views more easily). What’s true of Drupal is also (and probably more) true of many one click CMS.
One click CMS can implement thousands of features out of the box, but it takes a lot of work to customize the simplest element of design. On top of this, almost every answer to a problem in those CMS is... installing a module. You quickly end up with dozens of modules for a moderately complex website that would have taken no more than 10 modx addons. Why ? Because modx addons are like a lego kit : one modx addon can solve dozens of problems you face building a website feature. The fact that you can easily use the result output by an addon as a parameter for another addon opens many door. Flexibility improves how fast you can implement a website feature set and that is a key component of competitiveness. You have to experience this in real life projects to really grasp how empowering modx can be.
Another often mentionned aspect is Themes. One click CMS are meant to make it easy for people with little or no skills to set up a design by installing a theme. But themes have a rigid structure and the templates they contain are often fragmented. You don’t get any global view of what you’re doing. For me it’s a big disadvantage as a professionnal : I have never met a client in need of a "standard" look, what they want is to express their identity and singularity through their websites.
modx templates are built in such a way that you feel like working with static HTML while enjoying the power of dynamic content. No rigid structure, no php in templates but an easy, coherent and powerful syntax. The power of this approach is even more obvious when you need to control how the output of addons look, since modx addons can use chunks (reusable bit of content) as templates for their output.
I often say modx is template centric, while many CMS are feature centric, it might not be 100% accurate but it’s a least partially true.
Now you can’t deny modx has improved a lot, first with the install wizard module and revolution feature the web transport facility... but modx will never be a one click CMS and that’s good, because it’s more of a CMF anyway.
Last but not least, the question you asked to begin with : why is modx not number one ? Well, what is number one is my question ? User base is certainly an indicator that a CMS is successful, but successful does not always mean best... And best is relative to you needs as in "best fit". From day one, the aim what to provide unparalled flexibility to enthusiast and professionnal to whom it matters the most. In this area, I consider modx to already be number one...
I think revolution will open many doors and I hope to see some currently missing features happen, namely a comprehensive workflow module, a notification system, a file repository and a bunch of key addons to build community websites and or intranets extranets.