Different definition of "player?" I don't think mruby is meant to compete with MRI. JRuby is said by many to be a fork, and I would understand it to be a competitor, but I don't think their intentional focus is forking the language. The same goes for Rubinius. I didn't think of MacRuby to be a language fork. IronRuby isn't a player.
MRI, JRuby, and Rubinius were or are running significant populations of web apps. I wasn't aware that MacRuby was widely used for other than building apps on OS X.
Contrast this with Smalltalk, which at one point simultaneously had VisualWorks, Smalltalk Agents, Digitalk VSE (merged with other vendor), Smalltalk MT, IBM VisualAge Smalltalk, ObjectStudio, Smalltalk/X, Dolphin Smalltalk, and Squeak, all of which were actively being used to build or maintain desktop apps, and all but two of which were running on Windows. On top of that, there were significant semantic and syntactic differences between all of them, which make the differences between Ruby implementations seem quite minor. (Really messy and fundamental differences like namespaces and what does one expect Dictionaries to return when you insert something?)
Back in 1999 or so, most of the Smalltalk implementations were commercial, and many of the companies had no incentive to interoperate and some incentive not to. In contrast, I think there's more incentive to interoperate and avoid language forks for the Ruby community.
Also, if one considers the population of Smalltalk devs back then compared to Ruby devs today, then no, I wouldn't say anomalously many per capita. BASIC and Forth seem to be in a different league for number of implementations. One would have to do some digging, but it might be argued that Ruby has relatively few implementations even for its relative age.