The record-label wanted a hit for the album and they supposedly knocked it out in five minutes and it became a big hit all over europe (apprently only reached 61 in the US but it's still one of their most famous songs.

As for the other issues, just because the formula works one time doesn't mean it works the next. Just like reunions. Also, one should maybe view songwriting as "problem-solving"; maybe you already find the for *you* perfect solution to the problem so when you follow the formula the next time, you must settle for the second best solution; maybe a chord-progression that's similar but not so strong or a melody that is reminiscent of the hit but since you can't write the exact same song twice, you must change it a little which very likely leads to an inferior solution-a problem you obviously didn't have the first time when the formula was fresh.

Also, in the album-era especially, not everyone actually wanted hits (see Zeppelin who refused to release singles in the UK and still became mega); having hits can actually work *against* your brand. You might want to be viewed as a serious album-artist and attract that type of audience and the hits attract teeny-boppers instead which will lead to less interest from the original core crowd and since the pop-audiences taste is more fickle, there might not be an audience at all after a while...