AJ’s my favorite pony, but I don’t think she’d like being an alicorn too much. She doesn’t seem to me to be sensitive to issues of class or social structure, which is what you’d need to succeed in a aristocratic government as a princess. She’s too honest for politics, and she doesn’t like bending over for people or pretending to be something she’s not. Plus all of those snobby unicorn assholes in Canterlot wouldn’t give her any of the respect she deserves because she’s complete unknown and an earth pony to boot.
 
Applejack is an excellent leader, a better one than Twilight, but she’d be much more successful in a democratic setting than a monarchy. She’d like earning her position more than having it given to her. Christ, that’s actually what I’d love to see, Applejack leading the oppressed in a rebellion against the evil celestial hegemony, and becoming the new president of the New Equestrian Republic. That would require the monarchy to actually be evil though…

@boz
 
This

@boz  
One could argue that Rainbow Dash’s sonic Rainboom is a magical ability, considering she alone can perform it, and it is not produced through natural means (Wings and hair do not a rainbow make), and even Fluttershy has the ability to persuade lesser creatures with naut but a stare. Arguably, AJ is the only pony WITHOUT significant magical power. She’s awesome enough without it.

Every ponies of the mane 6 showed all qualities at least once and sometime more than their own.
 
-None of them is a compulsive liar, so all could be honesty except rainbow dash that give herself more credit than she should.  
-They are all funny when they try so all but flutthershy who doesn’t try much could be laughter. however, pinkie pie will alway be the best at it.  
-They never betrayed their friendship out of selfish purpose so they all fit for loyalty.  
-They all showed kindness at some point, though some doesn’t do it very much outside the mane six circle.  
-They all gave something in generosity, pinkie pie gave cake to the postman, RD gave tank his flying device, Fluttershy gave a pet to Rainbow Dash freely, Twilight gave Daring do book to RD, and applejack is explained on the post.  
-Only magic isn’t available for everyone as it require to have magic power, so only Twilight, Rarity and possibly Pinkie Pie with her control over reality can own it unless you count using enchanted objects like in the post.

Stupid theory:  
Earth pony genes are VERY dominant.  
Earth ponies are the bane of ponykind.  
Ever since the three races came together and began to interbreed after the founding of Equestria, there have been more and more Earth pony babies, many of who carry unicorn and pegasus heritage suppressed by their Earth pony gene.  
The more jovial pegasuses have gone almost extinct outside Cloudsdale, while the unicorns successfully maintained their gene pool through racial segregation.  
Applejack carries alicorn genes, manifesting as her many virtues, but she can never become a princess due to her Earth pony genes.

AJ would be a really bad Princess. Why? because she would have to leave her family. She couldn’t help them taking care of the Apllefarm etc.

ive never gotten this many comments on any of my art….

Yeah, but it’s not her destiny.
 
People keep making this excuse that “X should be a princess because whatever”, but it has nothing to do with that. It’s about destiny.

With AJ trying to lead it would be constant battle for power with RD.

@LanceOmikron  
I’ve always seen Applejack as the leader and Twilight the one who supports the rest, she can’t lead her team.

I’ve also felt that, should something happen to Twilight, Applejack would end up assuming the de-facto leader position.

@Background Pony  

Not bourgeois enough. No connections, no defeated villains.

Applejack does plenty of good stuff but that stupid hat ruins everything.

AJ’s a dirty earth pony. Case closed.

Exactly why Applejack is great, she has fewer flaws than everyone else in the series.

she is awesome already,just making a point.

Applejack deserves being princess much more than Twilight, especially that she didn’t have a rich family or capital city background for an easy start. Not that she needs it, AJ is already awesome the way she is, without pretty tiaras and other crap.

pretty strong case, there.

Hasbro’s marketing strategy will be Princess Twilight first, then all the mane6 shall be alicornified for the finale so they can milk the MLPFIM show and then discard it like some old dairy cow sent to the slaughter house.

Applejack definitely is the most mature of the mane 6 and has the strongest morals. There’s really nothing about her that makes her any less qualified to be a princess than Twilight, and arguably she might be more qualified.
 
(Not to mention that she is a commoner whereas Twilight Sparkle was already born into Canterlot’s nobility, which would have sent a better message to kids)

@Background Pony
 
Exactly what makes her awesome.

And Rarity has none of the virtues.

All hail.