First of all, none of those characters are nearly as iconic as Tommy. Tommy is THE Power Ranger, with Jason and Kimberly a step behind him. Jason because he was the leader, Kimberly because she was the first crush of many a youth.
I have no problem with them changing the ethnicity of some of the characters, especially when you factor in how originally, the black ranger was black and the yellow ranger was Asian. It's one thing to do that in the 90's, but there would be some serious raised eyebrows in this day and age. It was kind of necessary. The ethnicity change also really doesn't change the dynamic of any of the relationships.
Turning Tommy into a woman fundamentally changes the entire relationship with Jason. Not that that would be a terrible thing, but that's not Jason and Tommy. If their names weren't Jason and Tommy, I'd have zero issues with making the green ranger a woman.
Once again, that's probably unreasonable, but sometimes nostalgia is unreasonable.
I think it's also because the Tommy/Jason bromance was one of the better ones told in the original Power Rangers. There was a real rivalry between them, but also a great deal of mutual respect. They were both equals, friends, and yet also competitors. That made for a great dynamic, especially with Jason's guilt at failing to save Tommy from losing his Ranger powers. Many episodes were dedicated to just how similar, yet uniquely different, the pair were and how they had to overcome their issues and work as a unit.
Tommy was basically a darker, more powerful version of Jason, which is why he was both so threatening as a villain and so helpful as an ally (and why Tommy ultimately slipping into the role of "leader" later on was pretty natural).
While it's not inherently better or worse, swapping the gender does change that dynamic, and I think many would read into it far more as belligerent sexual tension more than friendly competition. Of course it could go any direction possible - that's the power of fiction after all, but I'll admit I have a deep and powerful nostalgia for the original Green Ranger (hell, there's a figure of him on my shelf right next to my computer as I type this).
If they change it, eh, fine. It's not like the original dynamic from the show ceases to exist.