Why did this show end after three seasons?

It was based on the Hardy boys books, and I guess they did all of them.

Low ratings I think. It was on opposite 60 Minutes on CBS and The Wonderful World of Disney on NBC, Sunday nights at 7. I don't think ABC has ever had a hit show at that time. Even Shaun Cassidy's teen idol stardom couldn't save it.

As a 11 year old at the time this show came out, I had read every Hardy Boys book at least a couple times. I wanted very much to like the show, but it never "did" it for me.
I really disliked the series because it was too "Tiger Beat" for me. I am not sure who they were aiming for at that time, but it didn't "hit" with me. It didn't help that of all the books that were adapted to stories, they essentially "ruined" in my child mind of the time. And Shaun Cassidy was like the kind of guy that your sister liked. I was listening to Aerosmith and Led Zeppelin at that time - and here they put this bubblegum pop star who sang that song "da doo run run" or whatever it was called, haha. So, I really think the failure of this show was because it either didn't calculate it's target market well enough or didn't do enough to appeal to other groups.

The first season was good,Nancy Drew even better than the Hardy Boys as far as quality was concerned('The Mystery Of Pirate's Cove' was my favorite).
The second season slipped up badly,with the shameful mishandling of Nancy Drew,reducing Pamela Sue Martin to "Guest Star" status on a series as much as hers as theirs.The stories teaming her with the Hardy Boys were silly,then recasting the character of George with a different actress,which is never a good move,making it worse with the bizarre reintroduction of Ned Nickerson,as if he never existed in the first place,which is outragous,and finally the recasting of Nancy Drew herself,after Pamela Sue Martin quit.
Janet Louise Johnson tried,and was attractive,but was up against it by still being miscast,and left holding the bag until she was phased out altogether by season three,which I haven't seen,and is still not on DVD.

This is why the ratings dropped:

"very few of the episodes of this series were based on any of the books"

Not to mention Shaun Cassidy got married and you can't have a teen idol get married. Kirk Cameron got married and Growing Pains was cancelled a year later.

Well, this is just my opinion, but they really screwed the show up in the third season. The show was always very loosely based on the Hardy Boys books, but the third season it wasn't even close.

But, I think the real reason it didn't make it was because it was going against 60 Minutes on CBS, and The Wonderful World of Disney on NBC(I think those were the two shows it was going against), and back then it was the Big Three networks, so going against those two shows, I imagine is ratings never really got as high as ABC had hoped.

I think they had the winning formula, at least for the Hardy Boys, the first season. I think they just needed to stay in Bayport more, and add some of their friends to the show, and at least let their mother make a cameo appearance, every now and then. Give us more of Chet, Biff, Tony, and Iola. But, instead, they went the opposite direction, getting rid of Aunt Gertrude, Callie (who I thought was a very valuable member of the team), and just spending a few seconds in Bayport, the second year. Do you realize, the only time they were in Bayport the second season, was when Fenton called Frank from Acapulco, when he briefly eluded his captors for a short time? That scene lasted about a minute. That's it, that's the only time they were in Bayport after what I thought was a good first season.