Swallowtails have been thin on the ground here too this summer.
Some observations...
Next to my garden/pool shower I have a good bank of Crithmum maritimum, and day-by-day I have been keeping an eye on many growing swallowtail caterpillars. Two died while tiny; moribund in the inflorescence; disease of some sort. Most of the rest grew to a decent size then disappeared - before they were full grown in most cases. So I think predation accounts for the loss of many potential adults (by agamas and/or birds and/or rodents?, methinks).
Of course, the caterpillars also feed on fennel, and I have not been monitoring their progress thereon.
Mike