Because it's not text. JPG is a lossy format and you didn't even tell us what the resolution was.
When you export a jpeg from Illustrator, you are converting from vector to raster data, and it's possible the text (was it 100% K to start?) is being converted to some four-color mix that "looks like" the 100% K, which normally prints as a dark gray on a press. You might also have your settings for handling black to set to output all blacks as rich black and be printing to a non-postscript device, in which case "real" black type would print darker, but your jpeg, being an image, probably would not be.
Why would you export jpeg from Illustrator to place into ID?
Text is about the worst thing you can ever save in JPEG format. It comes out really badly.
JPEG is not mathematically capable of representing sharp or hard eges. Here's a simple example from the Illustrator forum:
Here's a pretty simple image, and then that same image after it has been saved as a JPEG:
Notice the white squares are now sometimes gray, and some of the black squares are dark gray instead of solid black?
Now, it is certainly possibly to manipulate the JPEG compression settings to reduce this (but you have to know to do so, and almost nobody who uses JPEG does because, well, they wouldn't be using JPEG if they did...). But when you do that, you are basically telling JPEG not to compress. Which means you get huge files, and that isn't really the point. Other graphics formats can losslessly compress these kinds of images to a useful degree.
Text has these kinds of sharp edges all over the place. It's quality goes waaaa down in JPEG.This sort of thing doesn't matter much for photographs, which JPEG is designed for.
Save illustator files as Ai PDFs and place those.