If you look at the sound of a human voice on an oscilloscope display
you'll see a mess of differing frequencies and amplitudes... Human voices are complicated!
If you look at a morse code signal through an oscilloscope
you'll see sine-waves, only one frequency, and just two levels of transmission On or Off.
Once you try to piggyback these signals onto an electromagnetic radio-frequency signal
and send them through the ether, then there's going to be (1) interference,
(2) signal loss through attenuation, (3) jamming in wartime, (4) frequency bandwidth trimming
and (5) all sorts of other factors that intervene between the microphone of the transmitter
and the loudspeaker of the receiver...
As was said before, the human voice is a complicated thing
- and the human brain isn't very good at decoding the sound of a human voice if there's a lot of interference...
Morse code on the other hand is just about the simplest signal you can imagine
- and it's still recognisable and understandable when subjected to the same levels of interference.
So - Morse Code is more likely to get through and be understood than human voice...
A second reason, specifically for using Morse Code during the Second World War, was encoding...
Nowadays equipment exists to take an outgoing radio signal and automatically encrypt the data or voices during transmission, this is 'on-line' encryption... This generally didn't exist during WW2, so the best way of making sure your data was secure was by using 'off-line' encryption - I.e.: taking the text of your original message and encrypting the text itself (manually or mechanically) into an incomprehinsible set of seemingly random letters before transmitting the encrypted set of goobledegook... At the far end some guy would note down the serious of random letters, feed them through the same mechanical or manual decoding system in reverse, hopefully reproducing the plain text message you actually sent.
Again, using a system of dits and dahs to send individual letters is quicker and more accurate than sitting in front of a microphone and saying "hotel, echo, lima, lima alpha".
It should be noted that already by the outbreak of the second world war there was an alternative to morse - RATT or Radio Automatic Teletype, which basically encoded letters typed into a keyboard into a series of 5-bit binary numbers, which were then transmitted as Zeros ( a low pitched tone) or Ones (a high pitched tone), this was, in principle, far faster than morse code, but relied on a great deal of heavy and not necessarily very reliable electro-mechanical equipment to produce the signal, whereas morse needs just one man with a mechanical morse key to produce the signal and another with a Mk1 Ear to receive it.
Second:
During WWII, it was used for virtually all long-range radio communications
where teletype was impractical due to size, complexity or security.
You wouldn't really want such a heavy equipment set up on an aircraft -
one, it would weigh that much that you'd be carrying fewer bombs,
two, you really don't want it getting shot down over enemy territory and falling into enemy hands.
Yes, you could install a teletype machine on a warship, because alongside it, on the wall, there'd be a ruddy great axe
... And the last task of any radio operator, before abandoning his post, would be to reduce the machine to a pile of spare parts.
... But at the end of the day, morse was still simpler, more secure and more efficient.
Land-based use would be similar, communications between permanent HQs would use teletype with morse back-up,
whilst only short range tactical communications using a VHF transceiver would carry voice signals
(remembering that these voice signals, as mentioned earlier, would be un-encrypted and therefore open to the enemy).
And of course, teletype still suffered from interference and jamming in the same way as voice signals
... Nothing, but nothing, even today, can cut through interference as well as morse code,
nothing is harder to jam
and nothing is easier for the human ear to pick up and the human brain to decode.
Oh, and not forgetting of course, flashing light morse...
Rather than transmit radio signals
(which could be intercepted and DF'ed) ships in company, even today,
use a ruddy great spotlight with a shutter over the front to transmit morse code to one another)