The fourth letter of the Hebrew alphabet is "dalet" and not "daled." Why, then, do Ashkenazi Jews not pronounce it as "dales." I have heard an answer that "dales" sounds too much like "dalus", poverty but that doesn't make sense to me. Dalet sounds like dalut so nothing is gained. Is this indeed based on some religious consideration?
Here's my best guess: because when it's taught to little children, it's much easier to sing
Than to sing the accurate:
But both of those are far easier than trying to do:
If you end the middle letter with a fricative, you have to pause to breathe before the "h" in "Hei." Or else you'll call that letter "Sei."
And in Paleo-Hebrew, the symbol looked like a door. It's related to the word delet.
i'm not sure if there is a word that describes this phenomenon, but sometimes as the sounds of letters get lost, they get lost in interesting ways that show you funny things
So as Mori Doweed Yaaqob said in the comment, certain people have a tradition of saying dol/dhol. For the rest of us, the tradition is to spell it daleth, דלת. Now, according to scholars, and many older Middle Eastern Jewish communities, the sounds of ד and ת without dageshim are very closely related. They are made in the same place of the mouth, except one is voiced and one is not. In other words.
Think of the English word "the," say it to yourself and take note of how you pronounce the "th," that is the sound that many communities used to pronounce ד.
Think of the English word "theft," say it to yourself and take note of how you pronounce the "th," that is the sound that many communities used to pronounce ת.
And so when you have sounds that sound similar, some trail off, some replace others. And once people are used to spelling it this other way, then that way stays. So they were probably originally spelling it דלד with the daledh at the end being pronounced like the th in "the" and then once that sound was lost, they were left with the spelling of דלד and are now pronouncing it as a hard d sound.
In addition, many linguists claim that the more common Ashkenazic mispronunciation of the name of the letter ת is due to this same reason. The name of the letter ת is spelled תו. But since many Ashkenazim pronounce their v's and f's in the same place in their mouths (like the example above) but the v is vocalized and the f is not (like the example above) they mispronounce תו and pronounce it תף. i remember a Chabadnik getting into an argument over this issue as he claimed the letter was spelled תף and the only way to get the argument to end was to grab the Chabad siddur which as the letters spelled out in the first few pages to show him that it was spelled תו.