Question:
Why I could not use Int or long for this?
You could, if you size the payload correctly and setup the array correctly.
The issue is that on the receiver, you get an array of bytes, so reconstructing the int or long values is more difficult. Not hard, and certainly not impossible, but more complicated than you need.
If you want to deal with ints, highByte(), lowByte() and word() bear looking into. For longs, unions are the way to go.