I think you're being too hard on the guy.

Because some Japanese investment analysts have cited Dragon Quest X remaining on Nintendo platforms as contributing to Sony's stock price continuing to decline, alongside lower than expected television sales. It is true that is something of a blow for those (some investors included) who'd hoped Dragon Quest would return to the PlayStation family sometime soon--it remains one of Japan's biggest gaming properties in their domestic market. That being said, this is poor reporting from that Nintendo website--it fails to cite any of the other reasons behind that stock price fall. Square Enix's price has fallen because investors are concerned about Squeenix putting all of their most valuable eggs (in Japan at least) into one basket--Nintendo platforms, which are perceived by investors to be a poor choice right now. That, and of course the failures of FF14 online, are cause for concern for investors in the wake of this Dragon Quest X announcement.

To us, though, that all seems a little silly. It's always important to remember the majority of these investors and stock holders who shape the fortunes of companies on the stock exchange are only ever in this for short-term gain. In the long-term, Sony's gaming business is doing solidly on a global level, and has a promising product (Vita) near launch, Nintendo have acted swiftly to recover from the early failings of 3DS, and Dragon Quest X (despite being something of an unknown quantity right now) has a massive Wii base to sell into, is a guaranteed hit in Japan at least, and has the Wii U version to increase the life span of its online community, and therefore lengthen the period of time during which Squeenix will be able to make easy money through ongoing subscriptions. Dragon Quest X as an MMO on Nintendo platforms (which will also allow Squeenix to control the online structures of the game) is actually a fairly strong long-term move, if Squeenix have learnt from the mess that was FF14 online.

how stupid...

not like DQ has appeared on any Sony platform in the last few years

This.

WTF? This makes no sense.
It's like saying "Gears of War 3 releasing this September expect Sony stocks to drop even lower!"

When did 1 exclusive affect stocks?

See people don't use the same common sense you do. Investors would have reacted yesterday, and they did with the smaller decline, like 50 yen on the price of the stock. However todays result, is another story all together. Unless we are to believe that investors who generally are always quick to the punch have a delayed result and all started panicking the day after. Same applies to Sony and Nintendo. Even with the decline yesterday, it could have been attributed to patterns Nikkei, and not simply the news. 

Edit: However I do think, some investors are that slow, and maybe invested in either company based on "FANDOM" and this news hurt them. 

Sony stocks have fallen due to various factors (one of them being the incompetent that rules the IMF saying that Europe will go into recession again), but this news have had an impact(maybe small, maybe bigger than we think) as well because Dragon Quest is a trully monster in Japan.

Just to imagine how big it is, any DQ since DQ III has been launched in nonworking days due to the amount of people that didn't go to work or school when it was launched.

Now add that the last iteration of DQ on a home console was on the PS2, a Sony system and now on a Nintendo system and you can see why some can see this news as very bad news for Sony.

One game that's big in one country that wasn't supposed to release on one sony product of one sony division that could bring Sony maybe 20million of profit if it released but it's wasn't supposed to and will not is a significant factor compared to a weakening economy in Western countries where the whole of Sony sells it's luxury goods for which sales always take bigger hits than the whole economy? 

Hewhh  almost suffocated there…. Use more of these “.” Just a hint :P

Yes and no. If you look at it globally , this news is nothing to worry about for the things you said, but in the stock market every news is important, even more so now that there are so many bad news.

That means that if Sony, a company that sells luxury goods in a recession time, and which uses Yens, a currency that has become stronger than before making them lose money, loses one sure shot of money to improve their financial results (even if we already knew about this, the market has waited to see if S-E changed their minds or not), and that income goes to their direct rival, the stock market will sum that to the other news and causes Sony to lose even more.

Mind you, without this news Sony's stocks would have probably fallen 9.5% instead of the 10% that the link in the OP states, it's just that this has made things go a bit worse.