Why is it necessary to use the same restriction enzyme to cut 2 pieces of DNA that are to be joined together?

The reason why is necessary to use the same restriction enzyme is because

if we don't, the two pieces of DNA that we are trying to joint won't have matching ends and won't join properly.

To insert that gene we first cut him out from a DNA sequence (sequence 1), so To do that we use a restriction enzyme.

Meanwhile, we want to put that piece into another DNA sequence (sequence 2), so we also have to make a cut to be able to insert the gene and that will also be done with the same restriction enzyme.

This restriction enzyme needs to be the same one that cut the gene out of sequence 1. The enzyme recognizes specific sites in DNA, so when joined they will have ends that will match the gene's ends.