The primary reason would be money. If I recall correctly, the matching pool was at one point over $1 million, then it began to decline. TPF likely doesn't have the institutional willpower to keep spending at that level, and if they kept decreasing the pool, it eventually wouldn't be worth the hassle (the last time TPF did the match pool, the final match was 13 cents on the dollar). The second, and somewhat more cynical reason, is publicity. The Day of Giving was originally meant to drive business/interest to TPF, so once they milked as much publicity from it as they could, it became an afterthought.
I don't doubt that they genuinely hope the new format drives giving/exposure for a lot of organizations, but the cold reality is that if they truly wanted to maximize the day's impact, they wouldn't have changed the format.