It is because humans have much larger genomes than bacteria. The largest bacterial genome ; human clocks. But corn is about 3 times that, spruce is twice again that and some plants go even bigger. For example, every one of tulip’s chromosomes is about one human genome in size, and acteria are haploid, humans are diploid. That makes assembling the DNA fragments much more tricky, as you don’t want to have a final sequence that switches between the two haplotypes.