"Why do Christians have trouble with accepting evolution?" First off, I would carefully define evolution, as YEC Christians do believe in a type of evolution, as in a variation of a kind, so in that sense, all Christians accept that form of "evolution".
The real simple answer, because that's not what the Bible says, lol. The real big problem is that if Genesis is supposed to be some kind of allegory, it's the worst one ever thought of, it fails to the utmost degree at being an allegory or anything of that sort;
Bible - Earth created out of water
Secular Model of Origins - Earth created from hot molten mass
Bible - Earth before sun
Secular - Sun before earth
Bible - Oceans before land
Secular - Land before oceans
Bible - Light before sun
Secular - Sun before light
Bible - Land plants first
Secular - Marine life first
Bible - Whales before insests
Secular - Insects before whales
Bible - Plants before sun
Secular - Sun before plants
Bible - Birds before reptiles
Secular - Reptiles before birds
And the most theologically significant;
Bible - Man brought death into the world
Secular - Death brought man into the world
These two theories of origins COULD NOT be anymore opposed. The idea that Genesis is some sort of allegory or something like that is obliterated in just comparing the two.
Oh and
Exodus 20:11 - For in six days the LORD made heaven and earth, the sea, and all that is in them, and rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy.
I think that makes it even more clear in regards to Genesis, especially considering that verse is part of the ten commandments, not the ten allegorical commandments or the 9 literal and 1 allegorical commandment.
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Also, getting a better detailed definition/understanding of the words, using context, rightly reading poetic language as poetry, distinguishing literal from figurative language, understanding the perspective from which the text is written, and understanding when a figure of speech is being used, prevents issues of supposed flat earth verses.