Here are some earlier threads regarding this question. From what I gather, it's partly to execute a certain strategy in the "mainland" and part to avoid unnecessary complexity of when WI should be chosen as the Muster space.
- West Indies and BOTS
..and note what extra variables the British bot would need to check to get that timing: French Resources, Patriot Resources, and the number of cards played so far in the game (since Winter cards will be #7-11, 17-21, 27-31 etc.).
Also note that even with this good timing, it is only "tempting" rather than "must play". grabbing the WI gains the British 3-4 Resources (5 from WI, minus 1 for the Muster to place Regulars there and possibly minus 1 more for keeping Regulars in WI after the Winter Quarters), so these Resources must be weighed against what up to 6 more British Regulars in the Colonies could achieve...
- NP British West Indies
So, NP British will Muster in the West Indies if there aren't any Neutral or Passive spaces on the map, and the Muster can't take Control of any Pop-1 or larger space but can take Control of the West Indies.
This situation doesn't arise very often, and that is deliberate: while the West Indies can be lucrative, it is also a place where it is extremely easy to take lots of Casualties for little or no gain. In most cases, BritBot prefers to base its economy on City Control, Forts and occasional
piracy Naval Pressure.