To greenhouse or not to greenhouse

Ok, here's the problem-my bro says greenhouses don't work for seedlings/clones. However, almost all of his seedlings/clones have shriveled up and died-why?
Is it from over-watering, over-fertilizing or no greenhouse??
Attached is my homemade greenhouse, which i am growing 20 seedlings.
it is 2' x 1'.
my question is- does starting seedlings/clones in a greenhouse help or what?
thanks
check out my photo album!

I know this post was almost three years ago but that looks pretty cool. I wonder how it worked out.