Mostly because you shift goal posts to meet your criteria.
They liked the Tinker concept but there's no well-established Goblin lore character for them to use. That's why they used Gazlowe, because he's already been established. Same reason why they use Falstad for the Gryphon Rider hero in Heroes of the Storm, even though his looks are completely based on Kurdran rather than his Day of the Dragon or WoW concept. Heroes of the Storm takes a lot of creative liberties that way. Even Stitches was chosen just because Pudge the Abomination is so well recognized in MOBAs since DOTA. There's practically been a Pudge-like hero in every one of em.
Characters like Gazlowe existed before Blizzard went ahead to create new heroes out of units, like Brightwing, Lunara and Murky. Had Gazlowe not been one of the original heroes they chose (even back when it was called Blizzdota) he might not have been a Tinker at all. It could have been some new creation like 'Gabbo Blastwix'
Reforged simply ignored precedents of lore and threw in the latest designs and incarnations of popular WC3 characters into WC3, even if it was anachronistic. I already pointed out Grom Hellscream. Other bits include Kael'thas with green eyes (he had white eyes in WC3, Green Eyes was a TBC lore addition), Akama looking like the Broken, Dreadlords missing their tails, and Dagren the Orcslayer wielding Ashbringer (which was never his sword).
Reforged literally broke every lore rule in the book just to maintain a 'rule of cool'. It's simply not canon.