Why don't more people use longswordsmen or the swordsmen line in general?(exception Slavs)

I've found that longswordsmen + mangonels in castle age is an extremely powerful combo.Since longswordsmen are so cheap and create fast,you can consistently create mangonels and get up to imp.Then this becomes onager + champs.Add in trebs as well.As gold starts running low,switch to halbs.

Pros to champs+siege over halbs+siege

1)Halbs disappear to mass arbs or heavy scorpions.+3 champions dont

2) Champions actually have a shit ton of destructive power on their own,so if your someone like me with bad mangonel micro,you still do a lot of damage.Halbs do nothing except when against paladins,in which case you should start mixing halbs.

3)You need less vils on wood and can still sustain onager production and can also sell the wood.

4)Can make hussars/light cav by having more farmers for raids.

Best civs to use this with:Celts, Malians,Slavs, Japanese, Vikings (not as good as the other 4)