Some of the other answers describe the problem, but not the reason.
tl,dr: Data was nerfed to make him more appealing to humans. Humor, emotion and this minor language problem, possibly his love for humans was a trick too.
<PERSON>'s twin homebro, <PERSON>, is pretty much identical to Data. The thing is that he's a robot which is better than humans in every way. That made him pretty arrogant (he is made to be human, emotions included), and that made him disregard humans.
That and probably his tendency to lie, trick, make fun of people and demonstrate his superiority made the settlers on the colony he was made (by doctor <PERSON>, genius outcast professor) pretty fucking worried. They really kinda just wanted <PERSON> to turn the thing off.
<PERSON> was sad, of course, but <PERSON> contacted the "Cristalline entity" through kind of vague means. Of course that entity was far superior to humans (Source: Lore). Coincidentally it feeds off the lifeforce of humans, entire planets at the time if it can. It was also really fast, like warp fast. Cool thing. Offtopic more here.
<PERSON> contact the entity before he was disassembled. <PERSON> made his second robot with deliberately impaired features like his emotional response, as to not frighten the settlers. See also the uncanny valley, on why that might've helped. 44
Seemingly <PERSON> also fixed the extreme evilness that Lore had, possibly through introducing obedience or admiration of humans. Which would explain Data's goal of having the human experience.
So: Data was made not to use contractions by <PERSON>, possible just as an unimportant gimmick to make humans like him more. Along with other pinches in his capacities elsewhere, like his understanding of sarcasm, inability to laugh, etc.
It was shown that <PERSON> and Data have the same capacities other than in those areas where <PERSON> was "pinched". I guess <PERSON> is better as he's not adversarial and delusional, although it makes him illogical in many ways. The superiority discussion, of course, could be done later. One also wonders why there's only two prototypes, given <PERSON> still lives.
But hey, that's Star Trek. It works 3 steps of thought more than almost anything else, tells you something about yourself and society without talking about you, and gives you hope for humanity. But a step more the seams start to crack. Why isn't the teleporter all powerfull? Why isn't a superhumancomputerhybrid king of the universe (they actually did that one), etc. Offtopic again. Sorry again. I'm going to stop talking now. 44