The economic logic is that there are users who want it.
And, there are radios they can buy to satisfy their wants.
Here's an example of why I don't think the Japanese Big 3 will develop and market an inexpensive DMR handheld. Kenwood currently sells three, just three, handheld amateur radios in the U.S.--TH-K20A 2m analog; TH-D72A 2m/440 MHz analog and APRS; TH-D74A 2m/222 MHz/440 MHz analog, APRS, and D-STAR.
When the TH-D74 came out, the price was close to $600 and many hams complained about the price. The price has since drifted down to closer to $500 and hams still complain about the price. It's a heck of a radio (I own one) with a lot of features and an incredible full-color OLED display, but it's darn expensive especially when compared to a CCR DMR radio.
Don't you think that if Kenwood thought they could sell the D74 for $400 or $300 that they would? Since they don't, that leads me to three conclusions: their R&D and manufacturing costs are so high that they have to keep the price up to recoup their investment, their manufacturing capacity would not keep up with the demand if they lowered the price (Yaesu has been fighting this problem the past few months--they discounted the FTM-400XDR mobile radio and couldn't keep up with demand), or they just don't care to lower the price because the economics of sales volume, hence revenue, vs. cost is satisfying the accountants and executives.
At the other end of the Kenwood line is the TH-K20A at $125. A single-band handheld for more than $100. The CCR fan boys must laugh their butts off at that price.
So, Kenwood has demonstrated, at least, to me, that they
can't or
won't compete at the low end of the market. Could they add DMR to the TH-D74A? Maybe, and there were rumors a year ago that the CODEC in the radio might support it. But, they'd probably add another $100 to the price. So, in light of the fact that you can buy a CCR DMR handheld for, what, $175, it makes no economic sense for Kenwood to try to compete.
Both Icom and Yaesu have put a ton of R&D and marketing into D-STAR and System Fusion, respectively. They aren't going to undercut their chosen digital voice mode, which they both think are better suited for amateur radio than a protocol developed for land mobile, just to jump into the inexpensive DMR mud hole.