Well, there are a couple of points to make here...

Writers and artists are all different. Frank Robbins could write and draw a whole week of Johnny Hazard in three days. Milton Caniff, drawing a similar strip, required all week and at least one full-time assistant to draw a week of Steve Canyon. So that's part of the answer.

You can't quite add up panels and say that a strip guy is drawing X panels a week so that's equivalent to Y number of comic pages. It sometimes takes a lot of time to figure out how to tell your story in little two and three panel increments.

Someone producing a newspaper strip often has to spend a lot of time on the business end of things, dealing with syndicates, merchandising deals, newspaper editors, etc.

Also, a newspaper strip is kind of relentless. There's a deadline every day. There aren't many comic book artists who keep up the pace of consistently producing a specified number of pages per week. They do it for a while, take a break, move to other projects, etc. Strip artists have to keep doing it every week and that can wear on you.

Lastly (for now): With some, it's a matter of personal attention. If you went to Joe Artist and hired him to draw Batman, he'd do the best job he could but it would probably be the best job he could do and still get a page done in a certain amount of time because they'd only be paying him for that certain amount of time. If Joe Artist went out and sold a newspaper strip, he'd have a different stake in the game and more potential income stake in the strip's success. So he might try to put more effort and hours into that material. Most professionals do the best job they can but there are times when it matters more than it does at other times.

There are probably other factors involved. If I think of any more, I'll post 'em.