Specifically, instead of applying Title II to ALL broadband ISPs (FCC's position circa 2016) or applying Title II to NONE of them (FCC's position circa 2014/2018), why can't Title II only apply to ISPs in the scope of specific geographic markets where they are shown to be the only provider?
Because, as indigochild points out, the law doesn't provide the FCC with that option.
Actually, the law as indigochild quotes doesn't provide the FCC with any options; it just defines the term "common carrier" and directs the FCC to regulate common carriers. So how did the FCC twice reclassify ISPs? It used the legal concept of Chevron deference. Under Chevron, if Congress's definition of a term is ambiguous, and the FCC adopts a "permissible" reading of the law, the court system is bound to follow that reading. We know that Chevron is at issue here because the DC Circuit cited it repeatedly in their ruling on the validity of the 2015 order.
(We should also note that Chevron has come under sharp criticism from Neil Gorsuch, among other people. Were it overturned, the FCC's power here would be severely curtailed.)
In 2015, the FCC decided to interpret the definition as including ISPs. In 2017, it changed its mind. The problem with a pick-and-choose approach like the one described by the OP is that the law is categorical. It does not distinguish between "common carriers that are monopolistic" and "common carriers that are not monopolistic." Either internet service generally qualifies as a telecommunications service subject to common carrier regulation, or internet service does not so qualify.
So, if the FCC wanted to apply Title II to some ISPs and not others, it would need to make an argument such as "Some ISPs are engaged in interstate communications under Title II, and other ISPs are not engaged in interstate communications under Title II." But since all of those ISPs are providing technically similar services, it would be very difficult to defend this interpretation in court. Since Chevron only allows the FCC to adopt "permissible reading"s of the law, such an interpretation would likely be ineligible for deference, and would be struck down by the courts.
Now, once you apply Title II to all ISPs, the FCC can turn around and use "forbearance" to basically pick and choose when and how to enforce common carrier regulations. The process for doing so is described in 47 USC § 160, and is far more suitable to selective enforcement than using a convoluted definition of "common carrier." The FCC did use forbearance in its 2015 order to avoid applying some of the more aggressive regulatory options.
It is my understanding that the ISPs do not like the forbearance approach because it would potentially leave them open to heavier regulation in the future, particularly local-loop unbundling. They are concerned about unbundling because it would have the practical effect of damaging or dismantling their monopolies and oligopolies, as previously happened to the Baby Bells in the telephone market. As a result, they would prefer to have specialized legislation which would enshrine some of the more basic net neutrality rules into law (blocking and throttling) while closing the door on these more stringent regulations (especially those related to unbundling, zero rating, and paid prioritization). Congress has occasionally discussed doing this, but I think it is unlikely to happen any time soon, unless the environment in Washington abruptly gets a lot less partisan.