Because the price of iPads have been falling over time.

That could be because of cheaper production costs or because Apple has seen iPad sales dropping and traded some of its margins to sell them at lower prices. As a for profit company with very good margins, it only makes sense that Apple would continue to maximize that and not let it slip a lot, even if the gains may seem minimal to an outsider. It’s also the same reason why Apple continues to sell Macs at the same price a at launch time even years later without any hardware updates, even though Macs are a small percentage of its total revenues.