I don't have an exact answer, but would bring up 3 points to consider.
First, it is not clear that the term 'teaching assistant' has held the same meaning (paid graduate student position) over the long haul - from some delving into university histories the title has also been used for, well, people hired to assist in teaching various classes. In the early 1900s many institutions did not want to hire women as 'professors', so they got the less prestigious title of 'teaching assistant' and were the first fired in case of budget issues. It is also used in primary and secondary school settings where such personnel are not graduate students. The National Center for Education Statistics (nces.ed.gov) prefers the term 'graduate assistant'.
The Google Ngram for 'teaching assistant' looks like:
Meanwhile, the Ngram for 'graduate assistant' looks like:
'Graduate assistant' seems, to me at least, more precise of a term and more likely to indicate a graduate student helping teach a class (or do research - sigh). The difference in growth rate may indicate a shifting preference in language.
Still, the large growth in 'teaching assistant' usage needs an explanation. Lets look at National Center for Education Statistics data:
| Year |
Enrollment |
| 1899-1900 |
237,592 |
| 1909-1910 |
355,213 |
| 1919-1920 |
597,880 |
| 1929-1930 |
1,100,737 |
| 1939-1940 |
1,494,203 |
| 1949-1950 |
2,444,900 |
| 1959-1960 |
3,639,847 |
| 1969-1970 |
8,004,660 |
| 1979-1980 |
11,569,899 |
| 1989-1990 |
13,538,560 |
| 1999-2000 |
14,791,224 |
| 2009-2010 |
20,313,594 |
| 2020-2021 |
19,027,410 |
Since 1900, enrollment in institutions of higher learning in the United States has increased by a factor of 85. During that time the overall population increased from 76 million in 1900 to 329 million in 2020, roughly quadrupling. This means a roughly 20-fold increase in the percentage of the population attending college.
The post-war growth is quite striking, going from 1.5 million students in 1940 (before Pearl Harbor brought the US into WW2) to 2.4 million in 1950 with service veterans using the GI bill. (The GI Bill passed in 1944.) By 1960 the numbers were up to 3.6 million, with Korean veterans added under the GI bill, but college enrollment not yet affected by the post-war baby boom.
From 1960 to 1970 college enrollment more than doubled as the baby boomers came in. From 1970 to 1980 it increased by nearly 50% again. Growth slowed through 1990 and 2000, but jumped again by 2010 to over 20 million.
The post-war era also ignited significant government research funding to universities, particularly in STEM. For example, the National Science Foundation was established in 1950. This increased funding allowed an increase in graduate student populations, and this increase in turn provided a ready pool of new professors to accommodate the huge growth in enrollment.
Teaching assistants and graduate assistants have been around for 100+ years. Their number has greatly increased due to (1) a huge increase in overall enrollment, and (2) government support for university research.
