Mangas are usually written by one person, called the mangaka. The mangaka has to come up with creative ideas, original scenery, character expressions and dialogues while making sure that the story flow is coherent and planning ahead to pick up the story in the next chapter and see if he/she can take it from there. The mangaka first has to draw everything in frames of various sizes (some of which are very difficult to fill due to abnormal proportions [to create effects]), outlines everything and then fills in the ink. Sometimes the mangaka even has to come up with a color cover page/chapter.
Anime production houses employ many individuals who don't have to work around the clock all the time (shifts reduce workloads on individual employees). They already have most of the original artwork from the manga, and they simply have to digitize it and add vibrant colors (not that it isn't difficult). Most voice recordings are done prior to the final graphical rendering.
The mangaka is more likely to be pressurized when trying to meet the next week deadline than anime production houses, because a shortage of ideas is a disaster for the manga but easy to deal with in the anime (just adding a filler often makes it up, regardless of the quality of the content). This means that the mangaka prefers to spend a little more time on the story to think and draw more carefully.
The list goes on and on, but for the sake simplicity, I've decided to end it here. Just a side note that most mangakas work an average of about 18 hours a day (almost no days off), which can't really be characterized as lazy.