The loop does run over all T. Why do you think, that the loop stops?
A = zeros(1, 525600); % Pre-allocation, faster and simpler code
j = 1; % Assumption
for T = 1:525600
if T == B(j, 1)
A(T) = B(j, 2);
j = j + 1;
end
end
Equivalent code without a loop:
A = zeros(1, 525600);
A(B(:, 1)) = B(:, 2):
