The New York Times is the nations premier newspaper. Theoretically, this means that they have their pick of political cartoonists. Surely, if they're looking for conservative political cartoonists, they have a whole nation's worth to choose from so you can expect that they'll choose one of the best, most witty, most persuasive conservative cartoonists.
Either conservatives positions are so weak that even their best advocates look like shrill raving madmen, or perhaps the New York Times is purposefully not choosing the best conservative cartoonists. This is best case I can make for political bias on the part of the New York Times.