Becouse
1 Responder with invitation hcp must complete describtion of 65 suits exactly under 2nt.So only this way can complete this requirements.
2 with xyz style you always have chance to checback to find 4 cards of ♥ or ♠,so no need to reverse when responder has only 4 cards of ♥ or ♠
for exampl:
which one will you choice 1♦ or 1♠
1、if 1♣-1♦-1♥ how will you go on?and if 1♣-1♦-1♥-1♠-openner bid-2♠ then you showed your shape but this sequence is gf or nf?
2、if 1♣-1♦-1♥-2♠ and you must bid 3s to show your ♦6♠5 shape,but you maybe overbid with just invitating hcp
3、if 1♣-1♠-1nt after this sequence you can't describe your shape exactly,then both of you maybe miss 5/6♦.which one do you choice with xyz style if you 1♣-1♠-1n-2♣-2♦-2♠,you showed your hcp but your shape can't be shown enough
so how do you describe your ♦6♠5 with invitating hcp?
Responder must describe ♦6♠5 with invitating hcp under 2nt,becouse both of you maybe haven't fitting suit,but how do you realize it?
Maybe this way is ok,with xyz style
1、1♣-1♦-1♥-2♠=d6s5 invitation and nf
2、1♣-1♦-1♥-2♣-2♦-2♠=♦5♠4 invitation
3、1♣-1♦-1♥-2♦=gameforce checkback responder maybe with ♥4+ or ♠4+
Is it ok?