The reason it drops the T is that the word came to Russian from French (as did so many other words). And the French word "restaurant" is pronounced with a sile

The reason it drops the T is that the word came to Russian from French (as did so many other words). And the French word "restaurant" is pronounced with a silent 't' at the end, in the typical French way. So the cyrillic rendering of that sound ends up without a 'т' on the end.