@Puputanidya There are several reasons, some of them series-specific, some related to the format.
To begin with, webtoons are weekly, whereas most of the manga we do are monthly. That means manga chapters are normally quite bigger than webtoon chapters, thus increasing translation and edition time.
Second, in the case of our group, the contrast in edition difficulty is even bigger, because most of the manga we work on are quite hard to redraw.
Nana Toshi Monogatari needs time to proofread and to clean, as well.
Another factor is the translators. Webtoons are often done by Korean-Americans, or Koreans with a very good level of English. So they tend to be faster given that they're bilingual. One of our KTLs produced 40 chapters in two days. (Though she's an outlier even among Korean translators.)
In contrast, JTLs are mostly people who learned Japanese as a second language. We have three JTLs who are native Japanese speakers, and two of them are extremely busy and only produce a chapter sporadically. Many, like myself aren't even native English speakers, so we take significantly longer to translate a chapter. That's not to say all non-bilingual TLors are slow. One of ours is quite fast. (He happens to be the translator of
Rain Man, so in the case of that series it's just bad luck that he's busy sorting out real life matters.)
Other than that, some of our series are slow just due to bad luck. Several bottlenecks that affect manga don't matter to webtoons, like raw availability. Sometimes the raw provider is busy, sometimes we have to wait for new volumes to arrive for scanning etc.