Because it's easier to give new accounts an extra page compared to old accounts. With old accounts, you would either have to come up with eligibility requirements to receive a page, then you would have to distribute it, and then you would have a drawback from people who aren't eligible to receiving it. Otherwise, they would have to raise the limit to include everyone who already had the maximum amount of pages in the old system. Alternatively, they could come up with an option to have a free claimable rune page if you have less than 3 pages. I'm not saying that it's right that new accounts should be given more than old ones, but just listing some possible reasons, which really just boils down to laziness in all honesty. I personally use 9 of my 20 pages, and when it comes down to slight differences, the modifications are normally a single rune change.