Why was the Marvel Pearl more successful than the Marvel when the virtual gravity was more successful than the gravity shift?

My 2 cents (FWIW):
As I see it, the Marvel solid is a strong oil loving ball with a even arcing, more gradual transition and ball reaction. This type of ball reaction is no longer popular with the general bowling population. They want the ball to do the work that the bowler who used to work at improving his bowling skills used to be responsible for.

The Virtual Gravity had the skid/hook, then make a left turn type of reaction that bowlers now want to see in their balls. So that only proper drilling is required to get this reaction and not proper bowler execution.

The Marvel Pearl has a very strong ball reaction over all, as a pearl should but also handles more oil than most pearls, so it is a good step down from balls like the Virtual Gravity and the newer Virtual Gravity Nano. It is also strong enough to allow crankers who need to handle heavier oils and who can't use strong solids, like the VG and the VG Nano.

And I was wondering why the IQ tour is so popular even though it is a higher price points with the same cover stocks as the hot line series and the thunderline series

The IQ is closer in overall reaction to the Marvel Solid, but with slightly more backend, but not a lot. It's still relatively even reacting.

The IQ Tour is a control ball but with a more expensive core, even though it has been treated to have a lower flare. New core molds are very expensive to produce; so manufacturers would like to get their money's worth from each one.

There are still many people who don't need all the strength (ball reaction) of balls like the Virtual Gravity Nano (Vivid, IQ, Modern Marvel, etc) either because they don't face as much oil or their ability allows them to use milder balls and they want or need the control of a ball like the IQ Tour. Storm has not had a ball like this since the Storm Tour Power 11 years ago: a strong coverstock, a low RG and and  low differential:
http://www.stormbowling.com/products/balls/classic/tour-power?search=&exclusiveClassic=false