Here is the sum total of the article that actually talks to WHY the deal is blocked, and even then it isn’t very clear:

“An investigation was launched in Switzerland regarding RUAG’s attempt to sell Leopard 1 battle tanks to Ukraine through Germany’s Rheinmetall. This sale was halted due to concerns about violating Switzerland’s neutral status. RUAG acquired 96 Leopard 1 tanks from Italy in 2016 and planned to sell them to Rheinmetall, which would oversee repairs before facilitating their transfer to Ukraine. These tanks are currently in Italy. The investigation covers various aspects of the tank trade, including the circumstances of the original purchase in 2016 and the subsequent contract with Rheinmetall. Switzerland officially declined the re-export of the Leopard 1 tanks to Ukraine in June.”

Essentially just “it violates the law, and we’re investigating”.