Every government is going to have their own regulations for this sort of thing.
For the US, the law states:
Unless exempted under § 206.7, no drug product in solid oral dosage form may be introduced or delivered for introduction into interstate commerce unless it is clearly marked or imprinted with a code imprint that, in conjunction with the product's size, shape, and color, permits the unique identification of the drug product and the manufacturer or distributor of the product. Identification of the drug product requires identification of its active ingredients and its dosage strength. Inclusion of a letter or number in the imprint, while not required, is encouraged as a more effective means of identification than a symbol or logo by itself. Homeopathic drug products are required only to bear an imprint that identifies the manufacturer and their homeopathic nature.
In summary, you need to be able to ID a pill based on it's size, shape, color, and label. Anything else may be allowed but is not required.
I think if you printed a long name there would be serious limits on the possible font size, and this would make the pill difficult to identify. An abbreviation could easily be confused for a different drug; much safer for someone to have a pill that they can't identify and know they can't identify, than to have a pill that they think they've identified correctly but are actually wrong because they misunderstood the abbreviation intended.
Note also the requirement to identify "manufacturer or distributor of the product" - while you might think it would be more convenient for all manufacturers to have the same appearance for the same drug, that's not necessarily good from a regulatory perspective, where it may be very important that a pill can be identified as coming from a particular source if it is found to be contaminated or otherwise not conforming to specifications.