1. They ran out of pages.

2. Shazam!#34 and #35 have a drastic change in story and art, that would fit in better with the Shazam! material from World's Finest for a Showcase on Shazam! #2.

3. I've heard royalty payments change once you get into stuff published from 1976-on. The stuff in Shazam! #1 likely had only a few people to send royalties to. Add in the new team from Shazam #34-35, and the royalties may not have made this book profitable or break-even.