Why when some children loose their baby teeth do the new ones come out crooked, while some other kids' don't?

Crooked teeth. This is a great question but difficult to answer with limited space. Basically, there are genetic factors, environmental factors, and developmental factors. All of these factors affect us in some fashion. Bottom line though, we are all unique and different and it's beautiful! consult with an orthodontist to discuss options.
Jaw/tooth size. Genetics (and some environmental) factors determine the space available and therefore the mal-alignment of permanent teeth. Early intervention can sometimes be helpful in correcting this imbalance. One important factor is that the second baby molars are typically 2-4 mm wider than the successor tooth (2nd bicuspid), called leeway space. If preserved, it can change extraction to non-extraction.
Size. It's an unfortunate tooth-size vrs jaw-size relationship imbalance. Kids with no space between baby teeth often have crooked permanent teeth. See an orthodontic specialist. There are many interceptive orthodontic procedures to help teeth come in straighter and therefore reduce time in braces. Don't wait...Go now.
Yes. Certainly. Some folks are fortunate to have no orthodontic problems and others require treatment. I believe that majority of people have some degree of dental crowding or rotations - some need treatment; others, not so much.
It happens. No one knows, but yes this may happen.
Everyone is differen. Different bone structures, bites, jaw size, tooth positions, etc.