If your asking why fallout 3 doesnt allow it? mostlikey for memory overflow or just lag on the other hand...
Soo they dont let you have more than one
but if you want more than one
it is very easy glitching them!
there has been some patches on them but its still possiable
if your going to do it, then beat the pitt first..
When you travel to pit you lose followers and they go back to place.
look and read up here!
one easy one for to is..
Tell charon to work for you, or his boss i forget name...
now right when charon goes to kill him leave the place and go to paridise falls.
now but clover from him and head back to charon... charon will then talk to you and follow you!..
give them some tesla armor and there ya go!