I have a long theory or observation here.
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Chapter 1 - The Golden Era: When Flash Gaming was at it's Peak
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Around 2007 to 2010, web browser based gaming was at it's peak. There are a few reasons behind this:
1.) iPhones didn't exist until 2007, meaning that the smartphone gaming market was not existent or very small during that time (as it most likely took a few years for it to catch on).
2.) iPads weren't released until April 2010, according to Wikipedia. Again, this means no iPad or other tablet gaming.
3.) S.T.E.A.M. although released in 2003, probably wasn't as popular as it is now, back in the 2007 to 2010 period.
4.) The YouTube gaming scene was incredibly small. Let's Plays were in their infancy, and there were less classic game reviewers and less game reviewers in general making videos on YouTube.
5.) Reddit wasn't as popular during that time.
What this means, is that browser gaming was one of the few places that people could go to play free (or mostly free) games, many of which games had a nostalgic classic feel of older games from the 90s and earlier 00s. New developers probably created browser games to help flesh out resumes or gain some notoriety or because it was one of the few platforms in which they could share their creations to large audiences.
Yes, there was the Xbox Arcade, Playstation Network, and Wii had options, but I'm going to guess the barrier of entry, of getting a game released on that platform, was much more difficult than a hosting website.
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Chapter 2 - The Golden Era: Why Flash Gaming was at it's Peak
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What did this mean for browser based-gaming (a.k.a. Flash gaming). A few things.
1.) Flash games was one of the few platforms were new developers could reach large audiences.
2.) Many Flash games had a classic retro vibe to them, meaning people craving retro-gaming had few other places to go. Yes, someone might play on an old console you might have or use emulators. But, Flash games offered people new experiences, new games, but in the retro style.
3.) Flash games were almost all free at the time. (I say at the time because even though many are still free, you have more free-to-play style games now, with micro-transactions. I feel micro-transactions were much less at the time, if a Flash game had a pay option, it was typically a one time fee of $5 to unlock additional content.
Browser based gaming (or Flash gaming) was the place to be. It was the only place to fill those free, retro gaming needs, it was ubiquitous, and there was no where else to go to fill those needs. People played games on the websites, talked about the games and discussed strategy on the same website.
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Chapter 3 - The Golden Era: But it's the Golden Era for Smartphones and Tablets
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However, once smartphones, tablets, and S.T.E.A.M. took off things changed.
1.) For developers it offered an even bigger market audience than browser based Flash games.
2.) Smartphone, mobile, and S.T.E.A.M. games allowed indie developers to make those new retro style games on other platforms removing the uniqueness Flash games had over the current (at that time) console and computer gaming markets. In other words, if you wanted to play a new retro-style game, you could now use your phone, tablet, or go on S.T.E.A.M.instead of just browser games.
Note: Xbox Arcade and Playstation Network, as well as the Wii, offered actual retro games and new but retro-styled games. However more of them cost money, and they reached a smaller audience as you needed to own a console.
3.) Reddit become popular and so did S.T.E.A.M., both offering communities in which people could discuss games and game strategy, thus splintering the communities and funneling users away from browser based website.
4.) For developers, they could now charge for the games upfront or do micro-transactions with less hassle
Note: Micro-transactions can happen in browser games, but I want to say around 2011, it was more difficult on the user end as they typically needed a pay pal account. Charging through a phone or tablet that is already hooked up to an account is easier for the user).
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Chapter 4 - Why it's Not the Golden Era for Flash Games Anymore
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This is my theory as to why the forums are so slow. Flash games aren't as popular, for the reasons stated, and the discussion communities have been splintered, thus taking users away from the forums.
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Chapter 5 - The Golden Era: This is just a Sidenote, but I Wanted to say Golden Era One More Time to Keep with my Running Joke
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One last note, I know that Chrome now disables Flash by default and many browser games have moved to Unity using the WebGL support. Obviously the games are still playable in browser, but it really kind of marks how "Flash" gaming is ending.