Seasonal cyclical breeding in WTD may last as long as 6 months, until does become pregnant.
Also, consider that if you had 100 open does, not all will breed on a first 'exposure' (typically, in nature, only about 65% will 'stick' on first service)... so, about 35 are gonna be back in heat in 21 days... 35% of that 35 (~12) won't conceive to that service, so 20 days later, 12 are gonna be back in heat, rinse, repeat - 'til all, or most, get bred.
(From a cattle production standpoint, this is where some have opined that if you are achieving a 100% calf crop... you're spending too much money on feed.) And this doesn't even count the number that may spontaneously abort or abort due to pathogens, etc., and come back into heat.
IDK without doing some reading, when testosterone levels in bucks decline to the point that they're no longer fertile for that breeding season.
Add to that, as coachg says, northern WTD were brought in to some areas to re-stock... though I would expect that after 60 years, they'd have synched to the local conditions. But, I could be wrong.
I remember, as a kid of 3 or 4, in the very early 1960s, watching deer and turkeys be released into the woods around my Lee Co., AL home, as AL Fish & Game worked with the Lee Co. Rod and Gun Club to reintroduce those species and develop huntable populations. I'd say that they were successful in that endeavor.