"... Those who separate people in two kinds and those who don't." But I digress.
Well, because they can't. Duh. We all have differences and we find these to make our life more rich or more simple. Can't discuss with a guy who always agrees with you, can you? Or just image your better half to do as you do ... you couldn't even out your advantages and disadvantages! During work, we accept that people are different and we find that useful because it means that work can be spread and people can do what they're good at (instead of where they suck).
Sometimes, this difference goes deep. Way deep. It's so fundamental to our personality that we don't even question this. That's the fundamental schism which fuels the wars in IT. There are "vi" people and there are "emacs" people. Each member of both groups thinks the others are imbeciles who just won't see the light, no matter how often they got beaten some brains into them.
The "vi" people wants to get things done and they don't want the tools to get in the way. A tool should be like a hammer: Simple, to the point, easy to understand and use. If it comes with a manual, it's not a tool, it's a distraction.
The "emacs" people, on the other hand, like to have the most powerful tool they can find at their fingertips. They want to abstract, hide, build tool layer upon tool layer until the task at hand literally happens at the press of a button. If the tool can't be customized, it's not a tool, it's a nuisance.
No matter how much you wish for it, these two kinds of people will never use the other ones tool. If they have to, they will be constantly irritated. Take IntelliJ IDEA, for example. I'm a "vi" guy and this IDE just freaks me out. It's always doing something with my source that I never told it to do, always reformatting, always adding and removing whitespace, always getting in my way. I hate it.
Eclipse, on the other hand, comes with a rich tool set. I can have my source formatted any way I like, but it only does so when I tell it to. The default is to leave my artwork alone. Eclipse doesn't try to be smarter than me. Eclipse gets my jobs done when I want them to be done and it doesn't get in my way.
Don't get my wrong. I'm not telling you why Eclipse is better for you than IntelliJ, I'm saying it's better for me. I'm a "vi" guy.
Now, you may argue that I could probably hack IntelliJ into doing what I want. That's my point exactly: If I have to turn IntelliJ into an Eclipse clone to be able to use it, why not use the tool which fits my hand to begin with? And let's face it: No matter how customizable a tool is, after you've turned it into a clone, there will still be a lot of corner cases.
These come from the core of the design of these tools and that's what makes them as fundamentally different as two humans and no argument in the world will change that.
So, next time someone comes up and says "This or that would be better", answer: "It is better for you.. How about me?"