Ok then, I will put you out of your misery although by saying more power you actually answered the question (obviously without realizing it yourself). It is power that you need, and huge amounts of it! Faster ships required more power then engine manufacturers could actually develop.
Marine engine development did make giant leaps but because of the hydrodynamic properties of water the advancements that were made weren't very obvious. Why? How difficult can it be to increase the speed of a ship by one lousy knot!?
The answer is: very difficult. Let's take the Kaiser Wilhelm der Grosse as an example. Her two triple expansion reciprocating engines developed 31.000 shp powering two propellers. That power was sufficient to give her an average speed of 22.5 knots. If she however wanted to increase that average speed too let's say 23.5 knots then her engines would have to develop 25% more power to overcome the additional drag caused by the bow wave, something she was obviously not capable of. With the 33.000 shp engines of the Deutschland 1900 the limits of the reciprocating engine were reached. This ship suffered from severe vibrations.
The turbines of the Lusitania and Mauretania in 1907 developed 75.000 shp, enough to raise speed to about 25.5/26 knots. More power however also means more engines, more boilers, higher fuel consumption and the need for more propellers (4 instead of 2). All this resulted in a larger and therefore heavier ship which in turn caused even more drag.
And so a vicious circle developed: Bremen and Europa 1929, 100.000 shp, 28 knots. Rex 1933, 127.000 shp, 29 knots. Normandie 1937, 160.000 shp, 31.20 knots and it all more or less ended with the Queen Mary in 1938. To attain a speed of 31.69 knots she required a massive 200.000 shp! Only the continuous improvements in engine technology and especially power output could make this possible.
So, when I said: What was the cause of this disappointing achievement while other technologies seemed to make giant leaps? the right answer would have been that it wasn't a disappointing achievement at all, far from it actually! From 31.000 shp to 200.000 shp in 40 years, not to bad. Even today, 70 years after Queen Mary's record run, the large US carriers with 260.000 shp and unlimited fuel supply can't sail faster then +/-35 knots.
George, you're next.