Don't they just go with the companies that offer the best crews and financing after they make their names in racing? I don't know, which is why I'm asking. In really old news, I rode Ducatis in high school, and liked them just fine. They were my first two bikes, then I got a Triumph, and then a Norton.
The TT is different from any other racing series. Almost all of the riders come from Irish Road Racing (the Ulster GP as an example) and the World Superbikes actually prohibit their riders from doing any of the series.
So, manufacturers as participants, is a rarity. Instead these are all privateers, who modify the hell out of the bikes they use and the winningest riders are very sought after by the teams.