Isn't that because Apple is by far the biggest laptop manufacturer? Comparisons with the market leader are often useful.
Look at the little sliver of black that represents Mac computer sales (looks like ~5m sales/quarter) and compare with the orange line that represents Windows computer sales (just under 80m sales/quarter).
Even if 100% of the Macs are laptops and only 10% (as a pessimistic guess) of the Windows machines are laptops, sales are still heavily in the Windows territory.