Because the difference between full frame, APS-C, and Micro 4/3 just isn't that great for many applications. If your images are going to go onto a photo-sharing web site, corporate web page, eBay, or social media account, NO ONE can tell the difference. If you make 4x6" prints, NO ONE can tell the difference. Even at 8x10, it's difficult to see differences in a blind test.

Photography is a SYSTEM of components. The weakest component in the system limits the rest of the system to its quality. That weakness can be lack of knowledge, or hardware, or software, or a combination!

Back in the film days, you could have the finest Leica glass or Nikon glass or Canon glass available, and load Panatomic-X or Pan-F in your camera, but if you printed your negatives with an EL-Cheapo enlarging lens instead of an EL-Nikkor or Schneider Componon-S, it didn't matter. The enlarging lens was the weakest link in the system, so all you got was mush! (I know because I've been there and done that. When I bought the EL-Nikkor, my prints were several orders of magnitude sharper, more contrasty, and had much finer tonal gradation).

These days, most of the better cameras and lenses in any format, any brand, are excellent. They are far more capable instruments than most photographers realize, either because they don't know what they are doing with them, or they have serious shortcomings in the rest of their *systems.*

Is your monitor calibrated and profiled with a hardware colorimeter and matching software? If not, your monitor is dishonest. What you see will not be what is printed, or what is seen on someone else's calibrated and profiled monitor.

Are you adjusting raw images in decent post-processing software, on that calibrated monitor, in "proofing profile" reference to the printer profile you will use? If not, your output isn't honest.

Are you printing BIG BIG prints, on a pigmented inkjet printer, using OEM inks (or the very best knock-offs), photographic quality papers, and the right ICC profiles? Are you printing from 16-bit files, converting ProPhoto RGB color space to the printer profile in use? If not, you're not getting the most color from your files.

There are many reasons to buy a full frame camera, but there are just as many reasons to buy an APS-C camera or a Micro Four Thirds camera or a smartphone. Each has its uses. But when it comes right down to it, THE CAMERA MAY NOT MATTER. Spending $3000 to $7000 might make you feel good, but unless you have the chops and the rest of the system to reap the benefits of that purchase, fugeddaboutit!

If you balance your system and concentrate on improving your techniques at every stage, your basic technical image quality will improve. If you focus on developing a point of view, a vision, a sense of timing, a sense of composition, an understanding of light, color, balance, line, form, angle, perspective... THOSE things matter just as much or more than whether you have a full frame or smaller sensor.

When I figured that out, my world changed. I want to communicate visually... not fret over or brag about the size of my big chip or the length of my lens. (Hmm, reminds me of an old blues song I need to go play...)

I own both.
Yes money is a major factor but so is the much debated "reach" from the crop sensor. When using a high end crop sensor body you are also carrying less weight and getting that "reach" with a smaller, lighter, less expensive lens(or more reach from the same lens)
And there is a difference in the images. The crop sensor puts more pixels on the subject when used from the same distance and showing the subject to the same size. That said, the FF has advantages too, esp when the light gets dim.

Example, I used to use a 6D (20mp) and a 7DII (20mp). If I cropped so a bird was the same size in the image taken with both bodies then the 7DII had all 20 mp in the image while the 6D had 12.5 mp in the image. If I could use "foot zoom" to get the 6D image the same then the advantage returned to the 6D image. But if I took the 7DII to that same closer distance then it gave me a "closer" view again.
6 of one, 1/2 dozen of the other. Just change techniques dependent on the body I was using.
Now if I could afford the long fast teles (and was able to carry/hold that weight and size) I would stick with FF all the time.

I maybe from a different line of thinking because I lost my precious wife to cancer, almost five years ago. She was eight years younger than me. I would have traded places with her if I could. She was one of the finest human beings that I have ever met. I think of her everyday.

Her death made me realize that there is no point in drinking cheap wine or buying nothing less but the best camera equipment. BTW, make everyday as if were your last because there my not be a tomorrow. Just do it and enjoy life. Cheers!