Two words - product liability
Both Piper and Cessna were hit by punitive lawsuits back in the 80's / early 90's due to people killing themselves in their aircraft and, notwithstanding it was pilot error, the 'sue for anything' culture that was (is) prevalent signed their deathknell. Piper went under (that's why they're called the 'New' Piper Aircraft Company) and Cessna came close.
When the risk was removed by a change in U.S legislation the damage had been done and there was no money left in the pot for all new designs - just modernising the old ones as the industry is still recovering.