You can't really say. You don't know what was going on through their heads at the time, I just know from recorded history that some of the reasons were:

Kingdom building, Migrating to new lands, Being pioneers, Converting Religions.

__________________
██â€-█████████▒▓▓▓▓▓▓▓██████▓▓▓▓██████████████████ ██↑
██‡████████▒▓▓█████▓▓██████▓▓▓█████████████▓▓▓██ ██|
██‡███████▒▓▓██████▓▓▓███████▓▓███████████▓▓█▓▓█ ██|
██‡███████████████▓▓▓▒███▌████▓▓█████████▓▓███▓▓ ██|
██‡█████████████▓▓▓▒████▌▌▌████▓▓███████▓▓████▓█ ██|
██‡███████████▓▓▓▒█████▌▌▌▌▌▄███▓▓█████▓▓███████ ██|
██‡█████████▓▓▓▒█████████████████▓▓███▓▓████████ ██|
██‡███████▓▓▓▓▒███████████████████▓▓█▓▓█████████ ██|
██‡███████▓▓▓▓▓▓▓▓▓▓▓▓▌████████████▓▓▓██████████ ██|