While we're waiting for The Lozenge, I'd reckon it was because two sets of bike changes would present the possibility of twice the farce, twice the pit-lane collisions, and twice the re-entry collisions (Lorenzo/Pedrosa-turn 1-style).
And maybe twice the lap-counting ballsups...
As big a schemozzle as that race was, you didn't dare look away for a second for fear of missing the next drama : so much so, that ch 10 ran it ad-free for once !