Schmidhuber's disappointment should be with IDSIA or others in his network for not nominating him. The ACM does not itself survey the field looking for worthy candidates - the process is entirely driven by nominations which need to be supported by endorsements, solicited by the nominator, from heavyweights in the field. The maximum size group an award can be given to is three.

The nomination process is private, so it's not publicly known who nominated Bengio/Hinton/LeCun, but given the common CIFAR connection it might be a reasonable guess that someone there might have organized the nomination, maybe self-initiated with the goal of it reflecting well on the organization, or perhaps lobbied for by the recipients.