czasy past continous, past perfect

Dla czego w zdaniu: "By the 1920s, so may students were doing test each year that testing had become a multi-milion dollar industry", pierwszy czasownik jest w czasie past continous a nie perfect? Kiedy wiadomo gdzie wstawić perfect w konstrukcji z "by the.." ?

...students had been doing .... testing became..

czasy sa zastosowane okay, blady to brak liczby mnogiej tests oraz jednego l w million i ostatecznie brak hyphenu pomiedzy million a dollar
By the 1920s, so many students were doing tests each year (tu jest mowa o jakims trendzie, a wiec zrozumiale jest, ze byla to sytuacja zmieniajaca sie na przestrzeni czasu az do lat 20 i prawdopodobnie beyond that time)
that testing had become a multi-million-dollar industry (jezeli uzyto by czasu past simple, to zmieniloby sie znaczenie tego zdania, i oznaczaloby, ze ten dochodowy biznes mial miejsce przed jak i po latach 20; w obecnej wersji jest wiadome, ze autor chcial przekazac informacje o tym, ze ten interes dotyczyl tylko okresu przed, jak i zachaczajacy o lata 20 XX wieku)

>>ze byla to sytuacja zmieniajaca sie na przestrzeni czasu az do lat 20 i prawdopodobnie beyond that time)

..and maybe this is exactly why we should rather use the past perfect progressive here, don't you think?
Look, you give some background infortmation in the past for something that had been going on up until it turned out the way it did before another point in the past (and probably even continued up beyond that point ) - it became this and that as a result of that happening, right. I think we can agree on that.
This isn't exactly the temporal frame of habitual progressive in the past that can be implied here, but again, rather the iterartive sense of this 'pluperfect progressive', like a temporary habit up to some point in the past .
I might be wrong though, it's only one man's opinion.

the perfect past (simple or continuous), to me, extends its scope of time from a moment in the
past to another moment in the past, and stops right there; with the past continuous it can take a similar scope of time, up to a particular moment in the past and beyond it

I would agree on most everything you said except this one ' .. and stops right there'', because when the perfective progressive aspect combines with the past tense, one of its meaning is exactly the temporary hapit up to some point in time

yeah, I know you mean here
what I meant to say was something like, from that particular point it looks back on the whole process rather than providing a background

ok, like 'so many men, so many opinions', right ;) At least we know where we are, I mean we know that this all happened in the past, not in the future :)

this is how I see it in the form of a drawing:

________| = past perfect continuous up to some point |
had been doing tests

________|_____ = past continuous up | and beyond some point
were doing tests

obviously, like you pointed it out, in the case of the past perfect continuous, the situation may have continued beyond that point

agreed

right ..and with the effects of the happening being still visible

...so many students ......... that testing became

but that would be, again in my opinion, the best option in a sentence of this sort:
In those days, so many students were doing tests that it (eventually) became a multi-million-dollar industry.

with the one in this thread, I'd rather go for the form already applied:
By the 1920s, so many students were doing tests that it had become a multi-million-dollar industry.

Fair enough. You know, like Voltaire said ( allegedly )
"I disagree with what you say, but I will defend to the death your right to say it":)

Jeśli chodzi o literówki to przepraszam wyszły przez nieuwagę, oprócz jednej. Zdanie jest przepisane z ćwiczeń z odpowiedziami ( success upper intermediate), i tam nie ma hyphenu pomiędzy million a dollar. Także jeśli chodzi o czasy to na pewno są takie jak podałem, chyba że klucz się myli. Podsumowując jeśli dobrze zrozumiałem to past continous wskazuje na to że ta akcja jest tłem dla wydarzenia które jest opisane w perfekcie?

that's absolutely fine with me

nie koniecznie tlem, ale rowniez ukazaniem jakiegos wydarzenia jako zmieniajacy sie proces na przestrzeni jakiegos czasu; tutaj past perfect wynika raczej z uzycia wyrazenia By the 1920s

Proces, czyli stopniowo coraz więcej osób robiło te testy aż stały się multi-million dollar industry?

tak to wlasnie widze

odnosnie tego multi-million pound industry- nie ma sprawy, taka forma jest okay, podalem po prostu forme, ktore wiaze cale wyrazenie jako jednostke, w moim odczuciu i sposobie myslenia