This has been gone over time and time again. PS2 got all the support because Third parties were following the trends of the PSX success and it paid off. It came out first and it continued to be the most successful and so they threw everything behind it and by the end, it had 90% of third party support.
In this gen, the XBOX360 came out first and got a lot of third party attention, but others were hesitant to throw out all their support, waiting to see if the PS3 would follow the PS2s success. Mostly everyone ignored the Wii. In the end, the Wii and Nintendo surprised everyone and overtook them all by force. Many are just now turning around to join up, while others are pouting and refusing to make games for them due to grudges, disbelief or simply because they think they can't make money on the system/the system is outdated/the system is 'kiddy'.
In the end, the only two things that make the PS2 and Wii similar are that they are the low end systems of their gen. But Nintendo got the VERY short end of the stick and is still fighting a very nasty label put on them by both Sony and XBOX from the last 10 years. If anything, third parties are going to continue to ignore them for the most part until the next gen, until their wallets start barking at them.
Six upcoming games you should look into:
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