John VI Cantacouzenos falls of a cliff in October 1341, sparing the Byzantine Empire a long and devastating civil war. So he doesn't live to let Ottoman mercenaries into the Balkans. And the considerably stronger and more stable Byzantine Empire has enough time and resources to fortify Gallipoli and eastern Thrace in general to such an extent that the Ottomans abandon any plans of crossing over and turn their attention fully to Anatolia.
But the Ottomans were actually immensely lucky during their early campaigns in the Balkans. There are many points in the first few decades of their expansion when they could have been pushed back.