Brian Collins says “Probably because the protolanguage did not distinguish between those forms.”
Actually, Brian has sketched the answer in his response, but the foregoing isn’t quite it.
Indo-European languages often use notions of “morning”, “tomorrow”, and “early” interchangably. The Ancient Greek for “tomorrow”, aurion, is cognate to the Lithuanian aušrà “dawn”; and the Ancient Greek for “morning”, prōi, is transparently related to the word for “before”, pro. So it’s tempting to say “‘coz Proto–Indo-European”.
But (a) that doesn’t tell you why Proto–Indo-European conflated the two notions. And (b) it doesn’t tell you why Polish turned the word for “early” into the word for “morning”, as Brian reports. The Poles didn’t speak Proto–Indo-European.
Neither did Mediaeval Greeks, when they ditched both aurion and prōi, and instead started suing forms of taxia to mean both “tomorrow” and “morning”. Unsurprisingly, taxia comes from the ancient Greek word for “fast”, takhy… which in this context means “soon”, as in “early”: Remember: “morning”, “tomorrow”, and “early”.
If the same meaning shifts keep happening again and again, it’s not because Proto–Indo-European: it’s because those shifts make sense.
So: why conflate morning and early? Because morning is the early part of the day, duh.
So, more interestingly: why conflate morning and tomorrow?
If you don’t do something by COB today, what do you tell your boss?
“I’ll do it in the morning.”
What does that mean?
That you’ll do it tomorrow.
It’s even more true if you’re a peasant, like most speakers of most languages have been. When do you think of tomorrow? Not first thing in the morning; but in the evening, when you’re planning what you’re going to do the next day. What do you think of tomorrow as, in the evening when you’re planning your work? Not as tomorrow evening—that’s when you’re meant to have finished the stuff you’re going to do the next day. But as tomorrow morning, when the next day’s work starts.
So a lot of people would say a lot of times “I’ll do it in the morning.” In that common context, morning is ambiguous with tomorrow. So morning ends up standing in for tomorrow, as a more vivid or concrete way of referring to it.
And not because people have forgotten how to say “tomorrow”. Words rarely change to fill a gap: they change to make communication more vivid.