Before considering my answer below you should know that there are more efficent methids to do this. However as you question asks why this isn't working, my solotion focuses on the 'why' rather than computational efficeny. Learning is more important in my opinion.
So alterations can be seen below, esentially your code overwites the value of variable 'itime' in each loop, you never gave told matlab where or how to store multiple values.
Line 8 does this, creating a variable (itimeArray) the same length as frequency, the 'zeros' function dictates that each value itimeArray will start as a '0'. Line 14 then takes the variable 'itime' in each loop (i) and writes it to itimeArray of i. Thus gives a 131x1 double which can be plotted rather than the 1x1 overwritten double 'itime'.
To stress again there are computationally faster ways to calculate this but from a learning point of view I feel this is more intuative.
Let me know if you need any more help,
freq = 20:150;
maxfreq = 150;
tend = maxfreq - freq;
itime = 0;
itimeArray = zeros(length(freq),1);
itime = itime + 1./ (pstep.*freq(i));
itimeArray(i,:) = itimeArray(i,:) + itime;
ifreq = freq(i) + tend./(tend .* itime);
accl = 19620.*(sind(2.*pi.*ifreq.*itime));
title('Linear Steigende Frequenz')
xlabel('Time Step in sec.')