What disqualifies のみならず from being acceptable in this sentence?

Question

Below is a JLPT question that I'm struggling to understand

ロボットと人間の違いは何であろうか。1970年代に盲導犬ロボットや介護ロボットの研究開発が盛んに行われたことがある。しかしその当時は、技術レベルがまだ低かった○○、当事者たちに受け入れてもらうことはできなかった。目の不自由な人たちからは、「ロボットの研究者たちはわれわれの気持ちを理解していない」「われわれと心が通わない盲導犬ロボットの開発などはまったく意味がない」と非難された。また、病院の看護師さんからは、「ロボットには心がないからわれわれの代わりをすることはできない」ときめつけられたりもした。今ではロボットの技術も格段に進歩したし、医療、福祉に関する社会の助勢も変化した。しかし、基本的な問題が解決されたわけではない。生き物である盲導犬や看護師と人工物であるロボットとでは本質的な違いがあり、両者の間には超えがたい壁があるようである。

１．からこそ　２．だけあって　３．のみならず　４．こともあって

I understand why the answer is 4 but why is 3 not correct here? I asked a few natives and they could only say that it was unnatural. I have some grasp on the meaning of のみならず and have done some research beforehand but I can't settle on any conclusive reason why it doesn't fit here, only that こともあって is better. As the 2 clauses are stating facts it seems that it would be acceptable here, but why not?

Answer 1

のみならず means “not only” and what follows it is expected to be some addition that can be translated with “but also”. In this sentence, it would be another problem that existed at the time in addition to technology not being advanced enough. Robots not being accepted, however, seems more like a result of technology not being advanced enough than an additional problem.

Let’s suppose, however, that those two can be connected with のみならず as two problems, rather than a cause and a result, because that's not at all impossible.

What makes the sentence unnatural in this case is the particle that marks 当事者たちに受け入れてもらうこと.

Since what comes after のみならず is an addiction to what is said before it,も works well.

技術レベルがまだ低かったのみならず、当事者たちに受け入れてもらうこともできなかった。

が is not bad, either.

技術レベルがまだ低かったのみならず、当事者たちに受け入れてもらうことができなかった。

However, は gives an impression that acceptance by people, which was not achieved, is specifically focused in contrast to something that was achieved, and this doesn't quite go well with のみならず.

This problem doesn't exist with こともあって because ＡであることもあってＢ states B is the case, giving A as one of the reasons, and it is natural to put B in focus in contrast to what was stated earlier or implied by context.

In ＡであるのみならずＢ, on the other hand, A and B are supposed to be an equal pair and specific focus on B is not expected.

Answer 2

With a better look at the semantics, a rough parse of this sentence is going to be something like:

しかしその当時は、[技術レベルがまだ低かった○○][A]、[当事者たちに受け入れてもらうことはできなかった][B]。

「しかしその時は」 is a temporal adverbial elevated as a topic, so it's not part of the focus here. Let's take a look at the relation between A and B. If you use AのみならずB, you are putting A and B in the same category or on the same level. If the entire sentence contains causality -- though it doesn't have to -- A and B would be either 1. both reasons or 2. both results.

For example,

地震で家のみならず、家族も失った。

Both 家を失った、家族を失った are the results of the earthquake. The function of のみならず (のみか) is 添加 (addition, supplementation).

But in fact, with the semantics clear, clause A is a reason, among others, that led to clause B. There is a causal relation between A and B, and that's why you have to use こともあって. Here, こともあって functions to give inexhaustive reasons.