A fuel/air explosive does not bring its own air with it. It sprays the fuel into existing air as a mist and then ignites it. Where there is no air the fuel will not burn, so no matter how powerful or efficient you made this it would still only work for the first couple of minutes of a flight and after that the air gets too thin and you need a different kind of propulsion. And that is really not very useful, as you can see for yourself by trying to make a KSP rocket with jet engines as a first stage. (I imagine everybody as done this at one point or another. ) It sounds great on paper but you quickly reach an altitude where the jets stop working and you need to eject them as dead weight. So considering how little of the flight uses the jet engines, you might as well save yourself the trouble and just go with a conventional rocket for your first stage. Real rockets would suffer from much the same problem.