Bitcoin is not deflationary, it's inflationary, currently at a rate of 3600 new coins every day, this value halves every 4 years or so, so eventually around 2140 there will be no more new coins.
Well, yes. OK, that point was cleared up. But my original question was why not increase the rewards over time instead of decreasing as long as the end limit remains the same.
It'd give late adopters a reason to opt in?
The point is, Bitcoin in the original idea was supposed to emulate gold. And with gold back in the ancient times you have gold mines that had very good quality of ore (many ounces per unit of cost) that were first explored and emptied. New mines that were opened later had much less gold and thus the cost of production per one ounce increased. Same with Bitcoin - low difficulty and huge reward for the early (individual) miners, afterwards little reward, complex mining machinery and huge mining costs for these coming at the end, when there is very little of available resources.