For most of Mad's history, it was published 8 months per year. I believe they regularly skipped February, May, August, and November. I think things started getting changed around in the '90s (maybe it became 12 times a year), and I think now they might be only publishing 6 times per year (I'm not keeping up).
That's the same basic arc for my Mad reading. I guess it must have been for a lot of kids. My perspective was pretty limited -- I was the only kid I knew who read Mad regularly, so I was under the impression I was unusual. Later on I discovered no, there were hundreds of thousands of kids like me.
By the time Mad started taking advertisements, many of the core editors were long gone -- William Gaines had died, and founding members such as Al Feldstein had retired. Mad was owned by Time/Warner and was more of a "brand" than the cultural institution it used to be. I think it was a matter of taking advertisements or going out of business, since market tests showed kids were far less likely to buy black-and-white magazines, and the money from the ads would be used to offset the costs of color printing. I can understand being put off by the change, though.
You had a full run from the #1 comic on? That is impressive! Did you get all the Specials too? (Including Follies/Trash/Worst with bonuses intact?) That is very challenging indeed -- I still have many gaps to fill.
You know, as much as Mad has passed its prime, I still occasionally pick up new copies and find myself laughing. I bought a Mad before a long plane flight last year and had to hide my face from laughing so hard at one of the fake advertisements or something. Now, MOST of the mag isn't anything cutting-edge at all, but they seem to sneak in some stealth subversive gems every now and then. I also enjoy seeing Sergio Aragones continuing to contribute, although I hear recently he really hurt his back and has trouble drawing.