I think the relationship between a loa and their followers has many aspects…
There’s honoring the loa in general. Showing them respect and proper deference. Even an offering or two. I think all practitioners of Troll voodoo do this. More renowned or powerful loa get more respect, and lesser loa, less so.
There’s calling upon them for aid. A little prayer to Torcali over your crops or asking Bwonsamdi to guard the spirit of your deceased loved ones. Again, I think this is pretty common among most practitioners.
Then there’s actually invoking their powers in the way priests do. Effectively working miracles in their name. Having your wilting crops suddenly come to life with a green glow. Having the soul of someone who just died return. This is the kind of stuff I think is the domain of direct servants of individual loa.
Many members of the Druid class seem to be an exception, granted the ability to call upon multiple loa for their powers and serving them. Some are just servants of specific loa, whose particular gifts includes the ability to assume a specific form.
And this is drawing on my own headcanon, but I can’t help but draw on comparisons to real world Vodou, where the RELATIONSHIP between the lwa and the servitor is so important. The lwa aren’t just broad archetypical ideals. They’re actual individuals with whom you interact with as one would family, friends, or loved ones. You can have the spiritual equivalent of a godmother/father. They can be happy, pleased, or frustrated with you and vice versa. Hell, you can even marry a lwa.
One of the most important practices in Vodou is possession, where a priest invokes a lwa, who then comes to inhabit a mortal body. While they’re in mortal form, they will eat, drink, and even sit down and have literal conversations with people around them. “Hey Bob. It’s me, Mama Erizule. Got that champagne I like, you know, the fancy one? Oh, you got it! That’s so sweet of you! So I heard you were having trouble with your kids. Well here’s what you do about that…”
I like to think that since the Troll loa physically exist in the world alongside their followers that they have a similar relationship. And when your relationship with someone is this personal, you can’t focus on spending time and deepening your relationship with one without necessarily neglecting another to some degree. It doesn’t mean your relationships with the others aren’t important or meaningful, but it becomes increasingly hard to conceal your affinities.
In mechanical terms? It means characters are restricted to one loa at a time. Trying to take into account the complexities and expressing them accurately via multiple (sometimes contradictory) reps and relative boons accompanying each is just impractical.