The main reason enantiomers are emphasized more comes down to the interesting properties they have, particularly in biological systems.
A pair of enantiomers exhibit identical physical properties (e.g. melting point, dipole moment, etc) except for their optical activity (as you said, they rotate plane-polarized light with the same magnitude, but in the opposite directions). Chemically, however, a pair of enantiomers can behave very differently. This is particularly seen in biological systems, which are often adapted to work properly with only a particular enantiomer of a pair. For example, almost all naturally occurring sugars are the D-enantiomer and almost all naturally occurring amino acids are the L-enantiomer.
This has major implications in medicine; often times only one of the enantiomers is effective and the other can be inert or even harmful. The comments on your question mention thalidomide, a drug that was sold as a sedative and a treatment for morning sickness until it was realized that while one of the enantiomers had the desired effect, the other caused birth defects. Making enantiopure mixtures is necessary to ensure you are truly generating the effect you want (in the case of thalidomide, this still fails because the compound naturally racemizes).
For diastereomers, there is a greater expectation that they would behave differently due to their different physical properties. Hence, a lack of emphasis on diastereomers (which I'm not certain there is a lack of emphasis, at least from the organic chem course I took) may be due to the fact that the relationship between diastereomers is less direct than that between enantiomers and so it is tougher to draw conclusions about what properties they will share.