Foliated rocks are formed under a great amount of relatively equal pressure where as non-foliated rocks are formed under high temperatures.
Non-foliated rocks however lack this visual characteristic.
See the image below.
The reason for this is actually pretty intuitive. Foliated look like they’ve been smashed together for a reason: they have been. In the case of these rocks they have been under a great amount of pressure. Each metamorphic rock has a parent called a protolith. It’s the protolith that has undergone metamorphosis.
On the contrary non-foliated metamorphic rocks aren’t subject to the same degree of pressure. The amount of pressure that a non-foliated rock has pressed upon it could in fact equal the amount of pressure that the foliated rock had but it’s an unequal pressure so it doesn’t have the sandwich-type aesthetic. The key force behind the creation of the non-foliated rock is temperature.