Why do microarrays require a priori knowledge of the genome?
If the query genome is unknown, a microarray cannot be made for a target species. Microarrays have DNA fragments of what you want to amplify on them. Those fragments must be known.
From nature:

DNA microarrays are microscope slides that are printed with thousands
of tiny spots in defined positions, with each spot containing a known
DNA sequence or gene.

From wikipedia:

Scientists use DNA microarrays to measure the expression levels of
large numbers of genes simultaneously or to genotype multiple regions
of a genome. Each DNA spot contains picomoles (${10^{−12}}$ moles) of a
specific DNA sequence, known as probes (or reporters or oligos). These
can be a short section of a gene or other DNA element that are used to
hybridize a cDNA or cRNA (also called anti-sense RNA) sample (called
target) under high-stringency conditions.