why can't crossing-over between chromatids occur between heterologous chromosomes
There are two types of chromosomes that is Homologous and Heterologous chromosome.
Homologous means the chromosomes that belongs to the same pair, and in the other hand heterologous means the pair of chromosomes which doesn't belong to the same pair.
If we have a pair of chromosome A and pair of Chromosome B then one chromosome from A and one from B will be called as Heterologous chromosome.
We know that during meiosis maternal and paternal are next to each other and they have same number of allels thus the homologous chromosomes participate in crossing where as in heterologous its not the case.
In heterologous chromosomes the chromosome pairs are not alike they dont posses same shape and also number of allels and they are also far from each other thus they dont participate in crossing over.