It's a function of anatomy. The sublingual veins drain into the jugular and then vena cava. For drugs this avoids the first-pass effect, which is exploited when giving nitroglycerin for angina pectoris. Other ways to achieve this are suppositories (rectal veins) and intravenous (i.v.) or intramuscular (i.m.) injection.
https://en.wikipedia.org/wiki/Lingual_veins
https://en.wikipedia.org/wiki/First_pass_effect