Because the Air and MBP want to phase out in favor of MBPr and Macbook (plain) and they've been waiting on Intel to deliver anything significant to warrant a new product line for ages. (Intel Skylake CPUs in form factors and energy consumption compatible with the MBPr line have been TBD until recently --some still are--, and in limited production yields still. And previous generation didn't even give any performance speed bump of 2 and 3 generations behind MBPr CPU, and in some cases, worse). In the cases where Intel delivered, Apple did too (e.g. Skylake iMac and Macbook).

The Mac Pro has a similar story (and isn't their first priority or biggest seller anyway).

That said, not sure about any "multi-year gaps" regarding the iPad. The iPad Pro 9.7 came out in March, and the iPad Mini 4 less than a year ago, on September.