1) Because bacteria are too small to classify and it needs more higher microscope to do that. 

2) All bacteria have the same nucleic acid in there body, so those thing are very hard too classify because it was too small and all look petty same for that acid in their body. 

dont forget to thanking me!!

Classifying bacteria on the basis of their morphology is extremely difficult; bacteria are generally quite small and have simple shapes, though there are some bacteria, notably the cyanobacteria and actinomycetes, with sufficiently complex morphology to permit classification by shape. In addition to shape, bacteria have traditionally been identified and classified on the basis of their biochemistry and the conditions under which they grow. The advent of molecular biology has made it possible to classify bacteria on the basis of similarities among DNA sequences, and has revolutionized thinking in bacterial systematics. The cladogram above is based on DNA sequences that encode ribosome structure.