I started with PHP in 1996. While the community was not as large as it is today, it felt... a bit more alive. People were discovering this 'underground' tech, and to use PHP on a project was slightly rebellious, when you were supposed to be using Perl or ASP or maybe even ColdFusion or Java.
But the community changes as more people who are using the tech get involved. More people end up using the tech not by choice, but by mandate. Current infrastructure is LAMP, so new hires have to use LAMP, etc. PHP is no longer a rebellious underground choice - it's a/the standard.
Similarly, I was doing VB stuff in 1993. Rather revolutionary in many ways, and it felt like a whole new world. People were jumping ship from other platforms to be more productive in VB. I don't think it was ever 'underground' like PHP was, but certainly an active, vibrant community of people who wanted to be using that tech.
Fast forward to the mid 2000s - Rails becomes the new hotness, and people jump all over MVC. The people who are joining these communities in the beginning are there by choice. Lots of progress and rapid iterations are possible because of the passion and focus. A couple years from now, loads of people will be doing maintenance Rails projects, and the community of Rails people will be less active and less passionate about things than they are today. Many of the 'best and brightest' are moving on to node.js and other new hotness.
The cynic in me will point out that the newer members of the community coming in later will be stuck maintaining "first drafts" of attempts at Rails projects, that won't have benefitted from updates. I say this as someone who was shown a Rails 1.1 project in 2010 that had been started 3 years earlier, but never upgraded. I'm sure that's not the only one.
But... this is also the nature of tech communities. People come and go, but the people 'going' are often 'going' to the new-fangled stuff, and leaving behind a mixed-bag that the maintenance generation has to deal with. This skews the 2nd generation's view of the tech (not always, but some). I know people who left PHP for Rails, complaining about all the stuff PHP couldn't do. But they also left behind piles of poorly written PHP - there's nothing in PHP forcing you to write bad code, they just didn't have the discipline which something like Rails seems to force.
Long story long... the .NET community seems at a cross-roads, and the MS corporate involvement in the tech decisions isn't a spanner-in-the-works that most other tech communities have to contend with. Other tech - PHP/Perl/Python/Ruby/etc - tend to live and die outside the directives of one particular company. But the 'top down' community comment was spot on - it's hard to 'force' community on a group, and I suspect the reason most of the .NET users don't behave like other communities the OP compared to is that most of those .NET users don't want to be there. Not saying they hate .NET, but they just don't have the passion for tech that motivates the visible community members in other tribes.