Kurobas' popular, ongoing anime gives it greater reach than Ansatsu. That could change when the time comes for Ansatsu's anime.
Sales is a poor measure of reach and overall popularity. Book sales are a statistic, and like any statistic, can be heavily skewed by outside factors. Manga sales can be heavily skewed by age, gender, and intended audience. Sales of manga popular with these age groups can be skewed high. High sales would then reflect popularity within this specific demographic, rather than overall reach. Think of it like a political poll. If a poll is conducted predominantly among men 18 - 24, attending college, and living in a major city than the views reflected might not reflect the national political mood. Other demographics might not buy as much manga. Anything that appeals to these age groups might skew low no matter how popular. Nisekoi might not sell as much as Bleach, but the former has far greater reach at the moment. Nisekoi is only a few years old, with a popular, heavily marketed, new anime. The OP was a top ten hit this winter, peaking at #7 on the charts. Bleach's final arc is being serialized, it has not had an anime in two years, a new movie in four, little new merchandise, and no promotion. That is sometimes the best way to tell popularity. How heavily is a series promoted, and where. A few months ago I posted pics of life size Kurabas and Haikyuu advertisements in Shinjuku Station. Pizza Hut is currently doing a massive Love Live promotion. Pizza's come in Love Live boxes, restaurants and delivery scooters are decorated with pictures of the characters. Advertisement campaigns at the busiest train station in the world, and a national chain of fast food restaurants is real reach. Those are places where series can reach people who are not even manga or anime fans. Nothing remotely of the sort is done for Bleach. Those are the type of things that can help differentiate popularity. When the Gintama movie came out last summer, Lawson's (a national convenience store chain) launched a nationwide Gintama promotion. There was no comparable promotion for the Toriko movie, which came out around the same time.
If anyone is wondering, my feeling is that the Bleach anime ended because of time slot issues. Anime meant for early teen audiences traditionally have aired on weekdays in the early evening, after middle and high school kids get home from club activities. Over the past few years every channel that airs anime during that time has either been canceling or moving teen oriented anime. Most of the anime for early teens air on weekend afternoons now, except for a few hold outs, like Naruto. There are now fewer weekday anime time slots, most are filled by kids shows, like Cardight and Youkai Hunter. Bleach itself was replaced by a kid's show, the anime of the Rock Lee spinoff. I can't confirm if any anime airs in Bleach's old time slot now.