Their program's downward slide came long before they joined the Big Ten. The biggest blow was when the B12 wouldn't let them load up on, what at the time were called, "partial qualifiers." Osborne's success was built on a foundation of roided up lines, covering up the off-field shit of the skill players they were bringing in from around the country and some shady business of having all those in-state walk-ons getting their county's academic scholarships.
Also, in most years in the Big 8, they essentially had one game per year to gear up for.