Throughout the 90s, Def Leppard was much more successful in the US than Bon Jovi. Look it up on the Billboard website.

I guess Bon Jovi were than a bit more lucky with Crush than Def Leppard with X and this made all the difference. Both albums are cheap POP records, on both there is a song written by Max Martin, but one was successful and the other one wasn't.

Regarding the looks, I think Def Leppard also tried to adjust. In some clips/picturd Joe Eliot literally looks like an overaged Barbie doll.

another aspect is that Bon Jovi are American. What British band made it really big lately or is sill relevant? Maybe The Stones. U2 are from Ireland so they don't count. America has many more: metallica, aerosmith, chilli peppers, springsteen and even pearl jam.

I would add Jons looks and the fact that housewives are more loyal fans who also talk their kids into listening to the same music. In the 80s Def Leppard had more headbangers in their audience than Bon Jovi. Those guys are middle-aged men now who don't give a shit about Leppard, as opposed to the groupies who still have fantasies with JBJ.

I guess all of this explains it quite a bot. However, most important is in my opinion the come-back album. Crush hit big time. X failed.