Dio was at the peak of his popularity with the release of "Sacred Heart". Both of his first two LPs had gone platinum in the U.S. (1,000,000 + in sales) and were successful across the globe. Some sort of backlash was inevitable. Many were quick to pounce on the album for being inferior to "Holy Diver" & "The Last In Line". This is true. Whereas, those other albums had no filler "Sacred Heart" had it's share of mediocrity ("Shoot Shoot", "Fallen Angels", etc..). Additionally, the production is slicker and it's obvious the band was gunning for a hit. Most likely it was record company pressure. The Dio band's first two LPs were big sellers- going Gold within a year and eventually certified platinum as already mentioned- so likely record execs though if they could get a mainstream radio hit they could better the numbers. At the time it was in the insipid, sappy power ballad. Well, Dio wasn't going there thank goodness. I wonder if they tried to get him to write one.
The pop metal "Hungry For Heaven" had appeared on a movie soundtrack ("Vision Quest") and achieved heavy airplay on Rock radio. If ever there was a Dio song (along with "Rainbow In The Dark") that should've made the Billboard Pop chart it was this but no dice. It's just appalling that utter crap like Twisted Sister could get a pop hit but Dio couldn't. Actually, it's to Dio's credit- he was just too metal and history now looks back with disdain on TS and bands like Quiet Riot while the Dio albums still get respect.
That's the U.S. In The U.K., Dio was charting but it was a different single, "Rock And Roll Children", that was the hit. It managed a #26 placing on the pop chart. So, yeah, "Sacred Heart" and the following album "Dream Evil" were the closest Dio ever came to the cliche' "selling out". So perhaps there was more keyboard and more melody but that is not selling out. Just think of how nauseating Poison was/is and you'll release that Dio never strayed that far from his signature sound. He always had a sense of melody, not just songs with memorable guitar riffs (like the overly-esteemed "Iron Man" from Ozzy/Sabbath days)
Initially, this slightly more polished strategy worked as this became Dio's biggest selling album worldwide. But, for some reason Dio's fanbase began to dwindle shortly after it exited the charts. It wasn't grunge, which killed metal's popularity years later. It wasn't that Dio's songwriting skills wen't bankrupt ("Dream Evil" was a step up from this album). It's true the EP, "Intermission", wasn't well received but that's not it. What was it then? MTV's parade of photogenic Hair Band "Rock" bands plain and simple.