Why can the piece go straight?

@Vertonghen (1) Frisian is worthy to be learned if you get basic tactical knowledge in any other draughts variant. You might try to play Frysk! which is 5 checkers vs. 5 checkers endgame.
(2) Frisian draughts has own set of rules. It is not something unusual that in Frisian a simple checker can make jumping and capturing move in all 8 directions, but go only 1 square forwards diagonally.
There are some draughts related games where all 8 capturing directions are kept, but a simple checker can move forwards. One of them is Zamma glukkazan.github.io/checkers-like/zamma.htm . In that game instead of checkered board you see lined board, but these lines indicate checker movements and jumping and capturing directions. Zamma is very hard game (harder than Frisian draughts), but I believe that through this game you might get an understanding that additional direction would lead to more complications.
As well, to some extent Frisian draughts can be compared to Gothic draughts youtu.be/M4A-eKixxmw and Camelot game glukkazan.github.io/checkers-like/camelot.htm : these games share similar (but not same) jumping and capturing movements.

@kalnap thanks for the advice.