For many decades Switzerland was definitely the biggest fish to fry for most western countries regarding offshore accounts and tax dodging. By far.

There are but the prosecutors went for the biggest win, not the biggest target.

Making cases is hard because these things are very convoluted.

Swiss banks, however, were doing things which were really obviously illegal and their brazen tax avoidance made them a good target for prosecutors. Other targets are honestly just better at avoiding the law and will be harder to get to. Progress is being made though.