Firstly I want address some of your basis for the question,
Creationists themselves would strongly challenge the claim that Young Earth Creationism has only recently become popular, see here:
Also consider that James Ussher at least as early as the 17th Century attempted to calculate the age of the earth and came to a value < 10,000 years:
Now as far as your question is concerned,
I would challenge the claim that Young Earth Creationism is even today the popular view of Genesis:
"Table 1 demonstrates that of Americans in the 12 largest Christian denominations, 89.6% belong to churches that support evolution education!"
At best I see creationism being treated as a rather uncomfortable topic with most Christians, even if they believe it, preferring to not talk about it. Of course this is just personal experience.
The more specific you go, the less popular a 'literal 6 day interpretation of Genesis' becomes.
The most likely reason that Young earth creationists get the 'most' attention would be that they are in the most direct opposition to orthodox scientific consensus and are almost exclusively regarded as being on the fringe of science (indeed most secular proponents consider creationism patently unscientific, even anti-science or pseudoscience).
The reason that it may seem as if YEC was not discussed prior to the 20th (or 19th) centuries is because it may not have been considered a problem. Why would early church fathers and other Christian thinkers engage in a rigorous examination of the age of the earth if no one was really challenging the idea? It's hard to get evidence for a claim like this though because... well, if noone was talking about it, then we wouldn't have any evidence.
Here I think it is a case of absence of evidence is evidence of absence.
Another reason that creationism may appear to have gained traction post-WWII is that many of the Creation organisations that we're familiar with were only established in the 20th century.
Organisations such as Creation Ministries International, Answers in Genesis, Creation Research, Institute for Creation Research, have resulted in a more formal distinction between Young Earth Creationists and other flavours of Genesis interpretation.
See here also:
"In 1977, the first official creationist organisation in Australia was commenced-the Creation Science Association (CSA) of Adelaide, South Australia."
In 1961, Dr. Morris and Old Testament expert Dr. John C. Whitcomb published The Genesis Flood, the book that was widely acknowledged even by prominent evolutionary paleontologist Stephen J. Gould as "the founding document of the creationist movement."
This does not necessarily mean it has only risen to popularity in the 20th century. These organisations were established to formally challenge the rising tide of evolutionary biology in secular science since it was popularised by Darwin in the 19th century and then re emerged as the Modern evolutionary synthesis with the work of early 20th century statisticians such as Fisher and Haldane.
In light of the above sources mentioned, it seems reasonable to conclude that the evolution/creation debate in general has become a popular topic in lieu of the rise of the 'modern evolutionary synthesis' and 'strict YEC' at around about the same time:
Dobzhansky, Th. 1951. Genetics and the Origin of Species, 3rd Ed. Columbia University Press, New York.
By merging the traditions of Darwin and Mendel
The last part of your question regarding how DA and TE are
being considered heresy (or nearly so) by some literal 6-Day creationists
It's important to note that prominent organizations like CMI are emphatic that
Christians can still be saved despite believing in it [evolution]
However, they make their arguments based on a few main points, here are some examples:
- Jesus (according to them) affirmed the historicity of a literal 6-day creation
- Fundamental to evolution is that it is an unguided process (which is true!)
- Genesis (according to YEC) is blatantly clear on the literal 6-days
YEC came to the spotlight around the same time (or shortly after) the modern evolutionary synthesis was formulated, which makes sense.
YEC gets the most attention because it is the most extreme opposition to orthodox evolutionary biology.
YEC does not consider other flavours of Genesis interpretation 'heresy', however they do strongly criticize their views and defend a plain reading of Genesis.
I hope this provides some insight.