It's a bit hard for me to grasp why wouldn't be anyone interested in TSR after reading TSB.

Well, first there will be many people who buy or order TSB (or get it as a present) who will be disappointed when they read it, because they expected something different.

Furthermore, there will be many people who simply won't grasp what Satanism is or who can't identify with what Dr.LaVey wrote.

These large groups of people will probably not read TSR.

And even those who read TSB and find there is a name for their inherent philosophy and way of life do not necessarily need to read TSR, because not every Satanist practices Greater Magic and might therefore not be interested in a book about ritual magic.

Those Satanists, however, who use Greater Magic to obtain their personal goals, will find the basic structure of the Satanic ritual in TSB and therefore also don't need to read TSR.

These are possible reasons, as I see it, why TSR is sold not as often as TSB.
~ Suum cuique. ~