It probably has to do with the foundry generations. A12 and A13 are on the same 7nm [though TSMC does slight improvements.] Thus Apple can do a new chip generation for phones every year while the foundry generations take 2 to 3 years to happen.Apple seems to skip A##x generations? Like there was no A11x, I think?
In theory Apple could do new ipad A#x chips but it is a question is that really necessary for the iPad. Remember iPhone sales are 200+ million per year. The iPad sell roughly 40 million units per year [somewhere between 9 to 13 million per quarter] but remember most of those iPads are going to be using not the A#x cpus which you find in the iPad pros but instead the A# cpus either the most recent iPhone chip or a generation or two behind. For example the 6th gen 9.7" $329 ipad and the 7th gen 10.2" $329 ipad use the A10. Well the A10 is on TSMC 16nm foundries not the more recent 7nm foundries the A12 and A13 use.