To prevent tail buffet, reportedly:
In the NORMAL mode spoilers 5 and 10 are locked out during high-speed flight to prevent tail buffet.
That's from a study flashcard that was posted on chegg.com. But I could not corroborate it.
However, during the 777's flight testing campaign, tail buffeting did need fixing:
... Adjustments were still being made to the aerodynamics, particularly to cure some tail buffet [...]
— Birtles, Philip. Boeing 777: jetliner for a new century. p. 26.
Spoilers 5 and 10 are in-line with the stabilizer's tips, and a paper confirms the spoiler interaction, in general:
... separation over the wing may exist and also interact with the spoiler. Besides, the wake of the spoiler has to be accurately resolved, as it may be impinging the horizontal tail and trigger tail buffet leading to handling issues in some cases.
— Brooks, Timothy R., Gaetan K. Kenway, and Joaquim RRA Martins. "Undeflected common research model (uCRM): an aerostructural model for the study of high aspect ratio transport aircraft wings." (PDF)