This happens quite often to alot of people who like to role-play in a group. When hosting a role-play group, most people forget that it is indeed, a group activity. People like to join a role-play to act out their characters, interact with others characters, and have fun in utilizing their free time. However, if the group is simply three or less people having a say and subtlety ignore the other players in the group, it can appear tedious; as if the group is just a few people.
Also, to be fair, most players don't do that upon intention, as stated earlier by Lord and Snarf. Just the simple fact that some people don't know how to role-play as an active group. Then, there is the side of not knowing how to handle a group and upon other reasons.
If you are wanting to start a role-play group or think about it, just remember that you are hosting for other people to join in on the fun with you. It's not all about the "top" characters when you want new people to come into a story. It takes time to know how to play with others in various environments and it never hurts to give something new a try. Or, vice versa, just try again with a different approach and see what happens.