Probably because for every value of z it wastes a long time (a couple of seconds on my system) trying (unsuccessfully) to compute the integral symbolically, over and over again:
c = Integrate[3 (4 + 3 u/(1 + u)) D[Log[u + 1], u], {u, 0, 1}];
r = E^c;
dSymbolical[z_] :=
300000 Integrate[(1 +
z)/(75 (.3 (1 + u)^3 +
r (1 + u)^(3 (1 + (1 + u/(1 + u)))))), {u, 0, z}];
Timing[dSymbolical[.1]]
Timing[N[dSymbolical[.1]]]
