In each iteration of the for loop, t is a scalar. First t is 1, then 2, and so on until t is 50 in the last iteration. Now, after the loop is finished, t is still 50, so when you plot using t and I(t) you are in fact plotting only the final value of t and I.
You need to have all values of t available after the loop in order to plot correctly, so try this:
clc; clear;
t = 1:1:50; % t is a vector with 50 elements
for ii = 1:numel(t)
n=rand;
if n>=0 && n<0.5
I(ii)=0; % could just as well use I(t(ii)) here
else
I(ii)=2; % and here, since t(ii) == ii
end
end
stairs(t,I)
