Prolly cause the el'cheapo chains that come with these kits stretch so much and so quickly that they couldn't keep up with tensioning it...
Or, the problem I have with my new 41 chain - I need it to stretch just a hair more so I can pull another link. Technically I suppose it'd be two links lol, I'd use a 1/2 link to get it just right but the only ones I've found use a lil cotter pin instead of a peened over head - with the 41 chain on my bike, I haven't the clearance to trust a cotter pin.
I'm not sure I'd trust a cotter pin on my drive chain anyway o_O
So I'm in the unenviable position where if I pull the link I've not enough, leave it and I've too much... blarg... Should be ready this weekend though