Why is it easier to ‘tell’ rather than ‘show’?

I don’t know if other writers struggle with this, but sometimes I struggle to write in the ‘showing’ style, rather than the ‘telling’ style.

For those of you who don’t know, this is ‘telling’:

John and Mary were married in a beautiful ceremony. Gary was John’s best man for the day, and everyone thought Mary looked beautiful.

This is showing:

As John stood at the end of the aisle, he saw a vision of radiance beginning to move towards him. It was Mary. He had never seen her look so amazing. Gary, his best man, gave him a nudge, but he couldn’t tear his eyes off Mary to look at anyone else. Out of the corner of his eye he could see their family and friends snapping photographs and smiling with delight at the sight of her, but her eyes never left his until their hands connected.

Can you see the difference? The second paragraph is easier to read and sounds better. It certainly works well with novels. Even biographies and memoirs tend to opt for this style and I think it makes them easier to read. ‘Showing’ helps transport the reader into a story in a way ‘telling’ does not.

Why can it be so difficult to do? Sometimes I do still think it’s okay, especially if I need to explain something quickly but don’t want to bog down the main story with a huge scene devoted to it, but it should definitely be kept to a minimum, and it seems some days my head just doesn’t want to cooperate.

Maybe I should get a new brain!