I think getting angry or grumpy about it won’t solve the issue.
This game is and MMORPG and frankly, is plagued with a lot of issues so it’s far too complicated to pin it down to one or two issues.
So instead of talking about reasons I’ll give solutions 
1- Work on yourself, people that look good are often accepted ( gear / enchants / tmog / important items / good rio / good talent choices )
2- Communicate, I know it sounds ridiculous, but in my experience whispering the party leader and talking to them nicely can and will get you an invite.
3- Try to make friends, its a social game, using the very few keys you join find people you like, talk to them, add them on bnet and start playing together, this ensures you’re playing with people around your skill level and your preferred attitude = super healthy environment.
4- Be critical of yourself, like alooooooooooot of people don’t know their own standing and often excuse " reasons " for them not being taking, that is an wrong approach, actually try and gauge yourself, work on your flaws and better them and try to join content appropriate for yourself, it gets you into far more keys, if you’re not good enough for 15s, you can improve at 13s or 14s until you become good enough for 15s, skipping to 15s will only make life hard for yourself and will only get you backlash, don’t expect to be boosted for free.
5- Join a community of like minded players, there are multiple learning communities, pushing communities, helping communities and all different kinds of communities, they’re awesome check which one you like the most and feel like it suits you the best and join it.
6- Attitude, really it matters a lot more than people think, no body is gonna accept a negative person and hardly anyone will like them, tl;dr don’t be a party pooper, not getting accepted or finding people isn’t the end of the world, this is progress and its only natural that it starts hard and gets easier and then spikes super high in difficulty for this particular situation.
7- You’re better off not joining people that demand super high rio, they’re often bad and want to get boosted, you’re also better off not joining echoes farm groups, because those groups are made by particularly highly skilled players that want to get the dungeon over with super fast, if you don’t perform up to standard you’ll just get criticized and maybe even flamed by them, so unless you’re confident you are way too good for the content then you’re better off not joining these.