Because there is hardly any demand for that. Forgotten realms and a few more D&D settings are essentially watered down version of Tolkien's work without any depth and spirit. I consider Elder Scrolls universe using Tolkiens work as template as well and they have more depth and spirit. The problem is with role play gamers they hardly ever want deoth or developers with knowledged with classics, they mainly want escapism and Elder scrolls, Forgotten realms and other D&D settings essentially provides that.
Dragonlance was TSR's Tolkienesque setting for AD&D, designed around a series of 12 linked adventure modules forming an epic fantasy story in similar fashion to The Lord of the Rings trilogy, accompanied by a trilogy of novels, loosely following the adventure modules, that sold beyond the wildest dreams of anyone at TSR.
The Elder Scrolls setting, at least the human-populated provinces of Tamriel, is similar to D&D's Known World campaign setting (later renamed Mystara), in that it takes a pulp fantasy approach similar to Robert E. Howard's Hyboria setting for the Conan stories. Each country is inspired by real-world counterparts but freely adapted beyond the need for strict conformity, which allows for inspirations from different historical periods. Cyrodiil / Thyatis is based on the Roman Empire, Hammerfell / Ylaruam is based on the Islamic medieval Middle East, High Rock is based on Renaissance-era Britain/France, Skyrim / the Northern Reaches is based on Viking-era Scandinavia (the Known World has many more countries than Tamriel's four human provinces). Though this approach could be criticized for lacking originality, it reflects the diversity of the real world and permits for great depth, if that is so desired, since real culture and history can be drawn upon, or otherwise allows players to fill in the blanks from their own knowledge of history or pop culture.
Greyhawk was the more generic campaign setting, based on Gary Gygax's personal campaign setting, intended to be completely in line with the default, standard Advanced Dungeons & Dragons ruleset, rather more generic than Tolkienesque. After Gygax was ousted from TSR in October 1985, the management at TSR made the decision to shift focus towards campaign setting material, of which all that had been published were a brief "folio" in 1980 and a much larger World of Greyhawk box set in 1983. Although various snippets of information about the Dragonlance and Known World campaign settings appeared in their products, there hadn't been any products focused on the campaign setting prior to 1987, when a hardcover Dragonlance Adventures book was published and the long-running Gazzetteer series for the Known World setting began.
Since Gygax had initiated a lawsuit against TSR not only over ownership of the company but also over ownership of Greyhawk specifically, TSR had suspended publication of Greyhawk materials and decided it needed a replacement campaign setting, for which it turned to the Forgotten Realms setting of Ed Greenwood's personal campaign. Although TSR heavily sanitized its version of the Forgotten Realms relative to Greenwood's fetish-friendly original and also made attempts to have it be more realistic and historically-grounded than the original, the result was an extremely bland, generic campaign setting, which became the new default for AD&D and remained the standard even after TSR returned to regular publication of Greyhawk products in late-1988. TSR launched the Forgotten Realms with a box set in early 1987 and pumped out a vast quantity of campaign setting material, while intentionally overshadowing Dragonlance and Greyhawk, the latter of which was cancelled entirely in 1994, by which time the former was relegated almost entirely to TSR's much-expanded output of novels.
TSR also created campaign settings for its two attempts at creating alternate AD&D rules based on eastern Asia (Oriental Adventures in 1985, with the Kara-Tur box set released in 1988) and the Middle East (Al-Qadim began in 1992 with separate box sets for the rules and the campaign setting but was cancelled just two years later). In 1989, it launched the first in a series of unconventional campaign settings, quite distinct from the relatively conventional Greyhawk, Known World / Mystara, Dragonlance, and Forgotten Realms. Spelljammer was essentially AD&D in space but didn't sell as well as had been hoped and was cancelled in 1994 alongside Greyhawk. Ravenloft in 1990 was an attempt at implementing AD&D in the gothic horror genre rather than heroic fantasy and seemingly sold the best of the unconventional settings. Dark Sun in 1991 was post-apocalyptic fantasy, drawing inspiration from post-apocalyptic SF, incorporating the new psionics rules, and much darker in tone than any of settings except Ravenloft. Planescape in 1994 based itself on the existing AD&D planar cosmology, though with a new creation, the city of Sigil, prominently featuring as base for campaign operations. Birthright in 1995 had an underlying setting that was essentially in the pulp tradition of Hyboria and the Known World setting, but it rendered itself unconventional by shifting domain rulership from a higher-level endgame to the basis of play even for starting 1st-level characters, who were also granted a few special powers derived from their bloodlines.
