Why is fan-voicing Rainbow Dash (And Applejack) Difficult?

Before I start, I’d like to apologize in advance if this is in the wrong thread; it doesn’t have a lot to do with the show but it does involve two of the characters, so I decided to put it here.
 
Now, I enjoy a good fan-made pony video as much as the next guy, especially if they have voice-acting. However, I’ve noticed something that seems to be an occurring thing: when it comes to voicing characters like Pinkie Pie, Rarity, Fluttershy, and Twilight Sparkle, most people can impersonate them spot on. But why is it that Rainbow Dash (And to a certain extent, Applejack.)’s voices are so hard to mimic?
 
I searched voice impressions on YouTube, and watched fan videos that feature these two characters, and pretty much every one of them has the fan VAs’ take on their voices being inaccurate.
 
Now, some of you are probably going to tell me, “What are you talking about? There’s plenty of people who can impersonate Rainbow and AJ. You’re just not looking hard enough.” Which is true. I do remember a YouTube user named CreeperPaige who could do a damn good Dashie impression…but since her account mysteriously disappeared, I don’t have any videos of her impressions. (It’s a shame, too. She was the first person who I asked to voice in one of my Gmod animations. I did eventually find a replacement, but now that I listened to my video again, she just doesn’t sound like Rainbow Dash.)
 
So, long story short, why can nobody perfectly impersonate Rainbow Dash or Applejack’s voice?

@DragonBoi471
 
In Rainbow Dash’s case, it’s probably the raspiness. In AJ’s case, it’s probably the accent.

@DragonBoi471  
Define “most people” and “spot on”. Because with the ones I’ve heard, I can still tell the difference between them.

@Background Pony #F715  
Well, most people can impersonate Pinkie Pie(Wubcake), Rarity(AstroboyGF), Fluttershy(IMShadow87), and Twilight Sparkle(Rina-Chan) pretty decently-to the point that they sound just like they do in the show. However, with RD and AJ, it seems as though nobody can get the voices of either of those characters right.
 
As Jarkes mentioned, with Rainbow it’s her raspiness, and with AJ it’s her accent.
 
Hope this clears things up for you ;)
 
(Note: The names in parenthesis are VAs who can do the voices of those characters they’re next to spot on…as least to me, anyway.)

@DragonBoi471  
Wubcake is pretty spot on with a lot of characters. Personally, I love her Adagio and Aria, and she’s one of the better AJs, though still not perfect.