Because in FO3 (or maybe NV also), it's not just CHA, but *any* skill, perk, and attribute can unlock dialog choices. If your Big Gun skill is high enough, or Strength attribute high enough, or Medicine high enough, etc. etc. you have the potential to unlock more dialog. And you are even told how high of a stat you need:

"I can amputate your leg." [Stength=7]

"Let me give you a shot of this and you'll be fine." [Medicine=90]

"I'm not a doctor so good luck!" [Luck=10]

Yes unfortunately Charisma is the only thing that affects conversations. There are no other stat checks as you mentioned in this game. So basically if you give yourself 10 Charisma, you never have to worry about convincing anybody of anything again. In the entire game I have yet to fail a Charsima check.

That's unfortunate indeed. I gotta tell you, I don't like this. One big "dimension" of the gameplay is lost. Not every advantageous dialog choice has to do with charisma, you know. Now, this aspect becomes truly one-dimensional.

Yeah most people feel the same way. I feel the dialogue is the worst part of the game. No stat checks, harder to get info about things from people, no real unique options like being able to tell people if you are a member of the NCR in New Vegas. Mostly very straight forward, not like how Fallout dialogue is supposed to be.

You don't even have to do that. I am currently running around in power armor with 6 Charisma wearing a clean black suit and a formal hat which boosts it up to 9 Charisma. If I find some sunglasses with the 1 Charisma I will have 10 and can pass all the speech checks and that is without using any chems or booze.

Its on my admittedly very short list of complaints with this game.

It made spending points in other areas worthwhile knowing that there are checks for all of SPECIAL.

There are clothes which massively boost Charisma. I roll around in Power Armor until I need to talk and then out pops a guy in a suit