I canot answer for the designers of EnviMet, but in general using IEEE 754 binary format (that is also used by Intel) has the following advantages over ASCII:
- more compact representation
- no need for conversion, since the same format is used internally by the processor
- no loss of precision due to conversion from base 2 to base 10.
ASCII has in principle the advantage of readability by humans, but for such large datasets a visualisation software is needed anyway to ensure a minimum of readability.