While he never had actual screen time, we know enough about the circumstances of his childhood and how existing characters relate to him, so he can fit right in and interact with pretty much every major player in very interesting ways.
More importantly, far as I know, his faction has not really done anything of note in the books either, so it's not like they had significant material to adapt in the show. But the Golden Company is essentially an army trying to get dat bastard on the Iron Throne, so they will eventually do something in the books, otherwise why did Martin bother with that subplot? So using them in the show but reducing them to just a name, erasing all their book motivations and consequence of future actions, would make no sense to me. If you're gonna erase an entire faction from the books, why not just create a random mercenary army for the show?
I actually think that's the perfect way to introduce a mostly dormant faction that will only matter late in the game. Having not bothered with merely introducing them and randomly giving us a scene every other season to remind us they exist, they can now make it impactful and surprising, which is one of the things the show is known for.
Also, Cersei has no actual leading army figure on her side now, so there's no tension in the event of a battle. Storywise, a random mercenary guy placed against Jon and Daenerys is not interesting enough. Same with Euron, him and Cersei just don't click as a power couple and seem poised to fail, there's nothing of interest there if they win. But if it's "Strickland" & Cersei, their prospects increase dramatically, making a possible final confrontation very interesting (and an early defeat of the Walkers more passable).
Drax the Legend, Captain of the Fist, The Starwalker, The Jade Slayer