The simplest explanation would be that either the IQ tests are missing some aspect of human cognition that is getting more developed in spite of others, or that

The simplest explanation would be that either the IQ tests are missing some aspect of human cognition that is getting more developed in spite of others, or that the weighting used to calculate the final IQ number based on the particular results might be wrong or not representative of the actual cognitive effort required for each of them. Let's not forget that IQ tests are just statistical analyses, while we still lack a bottom-up explanation of intelligence.