What is the difference between them?
Also, how are questions converted to community wiki?
The short answer as to why one is open and the other isn't is timing. Gaming.SE is a pretty new site, and our standards and best practices have been evolving and changing a great deal over the past 6 months - particularly with respect to how we handle questions looking for recommendations, lists, and other 'repository' type questions.
The CW question you linked from a few months back was a (failed) attempt at reconciling some of the rather heated arguments that were going on at the time regarding whether game recommendation requests and other, similar questions have a place at Gaming.SE. It was flagged as Community Wiki in hopes that allowing wider editing (NB: It was posted back when editing posts that weren't flagged CW required 2000 reputation - there was no edit review in place at that time) would improve the quantity and quality of the content in the answers.
Lets be clear here: Community Wiki does not mean 'this question that wouldn't have been okay is okay because nobody gets rep'. It's been (mis)used that way in the past, and was made a mod only flag for questions specifically for that reason. The point of CW is primarily for answers. On questions that receive a wide variety of answers that might partially answer a question, a good CW answer synthesizing all of them is a valuable resource. Questions and Answers get automatically flagged as CW when they receive very large amounts of editing or from a great many people - this is done to reflect the fact that the community has taken ownership of the post at that point. Questions cannot be flagged as CW directly except by a mod however because, quite frankly, there is little to no good reason for a question to ever be CW anymore.
Questions are converted to community wiki by mods, plebians cannot tag them as such. Answers, on the other hand, can be marked as CW by the answerer, either when first posted or later. If more than 5 people edit a question or answer, it can also automatically be marked as community wiki.
Denoting a question CW is usually performed for more subjective questions, or those that will by their nature change over time. As for your two particular examples, the open, CW question is permitted as it is a useful question that is where we can continually add information in order to have some response for those seeking game recommendations. The closed question essentially solicits a list, which while possibly useful, is currently "off-topic" for Gaming.SE.
We have provided some additional guidance at the blog:
Most of the time, you should be asking yourself “How can I improve this post so that community wiki isn’t needed?” Community wiki is like a cheese knife: it is a specialized tool to be used sparingly, and only in very specific circumstances.