Maybe because this powerset is stronger than what the old jean had. teen jean is not holding back. she went on a head on collision with gladiator. she wasn't even trying to save anyone. she was just trying to prove a point then. Old jean was always careful and didn't really realize the full potential of her powers till she started embracing it during new x-men.
The argument now is, which is more powerful, jean's full psychic potential "XORN" or Jean's Phoenix state (new x-men)?....Both were just shortlived.
I think I have a problem with that, keeping in mind that I loved New X-Men's incarnation of the character. But I do think one of Morrison's miss-steps was how he handled the Phoenix. He fell into the same trap he had criticized other writers for - ignoring 20+ years of the character's history without the Phoenix, and making this another revisit of the same old concept.
This goes back to a discussion I had with other people on the Vine. I think the idea that Jean was always careful and holding back came from three different periods in her life, only one of which was canon. She was holding back as a teenager and started to let loose by the time her telepathy was unleashed (this was said to be released when she became Phoenix, but depending on if you follow the Phoenix Replacement or Jean was a Phoenix Host explanation, that can go either way), then the '90s X-Men cartoon reinforced the idea of a careful, passive Jean, and the movie set up a similar character trait.
But for the majority of her history, Jean was always a hot-tempered atom bomb with a heart. I'd say that writers generally justified her various power boosts as a result of her holding back, but she's generally been portrayed as far more powerful than Teen Jean's current powerset (which seems to amount to nothing more than taking psionic energy in and returning it as telekinetic force).
I don't know - we'll just have to see where they take this character. I've read most of the original X-Factor run, and the scale of telekinetic feats she accomplished in it was extremely high, and she had a lot of great feats during the '90s-'00s comic era which hinted at her ultimate potential (couldn't find the panel where Jean survives the Sentinel attack by body swapping into Emma's body, and the panels from the issue(s) afterwards where Prof. X explains that all psis of her caliber have the capability to exist as psionic energy) without the PF so I don't really think introducing a new powerset was that necessary considering TP and TK are rarely well-explored anyway. I'm more concerned with character development than powersets, and I haven't seen much from Teen Jean TBQH despite all the attention she is getting.
The "holding back" argument especially sounds ridiculous considering some of the more powerful mutants need to do this, yet they don't get this brought up every time they need a power boost. Storm holds back on an extremely neurotic level because losing control of her emotions could devastate the area around her - we've seen her let loose and tap into her full power reserves, yet it's never treated as if she is weaker or less powerful when she is more disciplined when using her powers. I'd actually suggest that this is one of the traits that made both women such good friends - they liked their powers, but unlike others, their powers were always "on." But that aside, it just irks me that Jean is always treated as a fainting goat without a massive power boost.