Because I got tired of the blue screens of death, and despite the MBP being way over priced it does one thing very well... it works 99% of the time. Plus it looks cool, if you care about that sort of thing.
Been using both for years, at multiple orgs, and can say with confidence JIRA does suck. Everyone in my org had given up trying to use it, I went through the pain and don't wish it on anyone... then there's Trello, it just works expected.
With JIRA, prepare to invest significant pulling your hair out if you want to use any features. It's amazingly difficult to enable the use of the Trello-like board feature.
Go ahead, try enabling the "Agile" feature and see, it doesn't work out of the box!!! Have fun digging through Google to figure out why you're getting cryptic errors, only to finally figure it's because....
Oh, you have to set up additional permissions to enable it.
Oh, and also enable a certain field (epics) but this needs to happen in the back-end, as it's some SQL query to enable it.
Oh, but you want to sort issues by drag-drop like a real board? Nope. First go find the right permission to actually enable sorting!
Oh, but it still wont let you sort! Did you forget to 'sort by ASC' in your filter query? Otherwise, no drag-drop sorting for you!
Why those things aren't just enabled when you created the board in the first place, boggles my mind!