Note: I am assuming from your question that you are based in the USA, and the answer below is based on this assumption. The answer below also contains some conjecture, but given the secretive and byzantine nature of most streaming/airing/export arrangements, I suspect this is as close of an answer as we're going to get using public knowledge.
The Great British Bake-Off, like many BBC imports, was originally aired in the US on PBS. PBS began airing the show in early 2015, after the highly popular fifth series aired in the UK:
The October finale was 2014’s second most watched TV program in the U.K., bested only by the England-Uruguay World Cup match.
However, by that point the show had been airing in the UK for some time. In its first few seasons, it was not nearly as popular; the first series only had about one-quarter of the viewership of the fifth series. Given these viewership numbers, it is understandable that PBS decided to air the more recent, more popular, and more "matured" Series 5 as "Season 1" in the USA, rather than airing Series 1. (By "matured", I simply mean that the format of the first series was rather different than the format later on.)
Subsequently, PBS aired Series 4 as "Season 2", Series 6 as "Season 3", and Series 7 as "Season 4". PBS has also announced that it will air Series 3 as "Season 5" starting in in June 2018.
As far as Netflix goes, they can currently only stream the show via an agreement with PBS; and PBS's plans for airing the show in the future (other than the above-mentioned airing of Series 3 in Summer 2018) are as yet unknown:
PBS, which also licenses “The Great British Baking Show” to Netflix, said it had not yet decided whether to air future seasons in the United States.
Adding all of this up, it seems plausible that PBS effectively has the rights of "first airing" in the US for Series 1–3 of The Great British Bake-Off. This would make sense given the long-time business relationship between PBS and the BBC. If this is the case, then it seems unlikely that Netflix will be able to stream these series until after PBS has aired them. In particular, it seems plausible that Series 3 ("Season 5") will appear on Netflix some time in the fall of 2018; beyond this, it is not yet clear when, if, and where Series 1, 2, and 8 will air in the USA.
EDIT (Sep 2018): Netflix has now acquired the rights to show Series 8–10 of The Great British Bake-Off (the first three of the "Channel 4" seasons) in the US:
- Serve 10 new episodes at once on Netflix in the US on August 31, 2018 (Netflix collection 5, which is UK [Series] 8)
- Assemble 10 additional new episodes to be served later this year (Netflix collection 6, which is UK [Series] 9) and in fall 2019 (Netflix collection 7, which is UK [Series] 10)
Note that UK Series 9 was being aired in the UK at the time of this announcement, and Series 10 had not yet been produced. It was subsequently announced that Netflix USA would release "Collection 6" in the USA on November 9, 2018.
Confusingly, PBS also aired Series 3 (2012) over the summer, under the name "Season 5". Netflix will eventually start streaming this as “The Great British Baking Show: The Beginnings” in late 2018.