Because people expect rangers and pali's to fight. People expect clerics to heal. Not usually do you see a ranger answer a LFM asking for a clr/fvs saying they can heal the group. I can roll up a BC and probably play it pretty well because I already know how to set my spell selection per quest, people on the server to run with, etc. Someone new to the game who is running things for the first time doesn't have this knowledge.
Here's an example of what I'm talking about. Was running Hazadil's the other day. Came time for the swim through traps and the rogue gets murdered in the first tunnel on norm. So someone asks, what's your reflex score? His answer? 8. At level 14. Now is this build viable? Of course it is, just not to fill the trap/evasion slot. That's good knowledge to know before a group walks into a quest. Would they still have taken him? IDK, but he didn't feel good about it. He didn't know what his reflex was supposed to be or that dexterity was even important to a rogue. People should play what they want, I completely agree. But there are certain roles we play in DDO. When you go outside of those roles it makes people wary of you unless they know you are a solid player.