Because the story that started in S1 continues in S2. Haven't you seen how TV series are numbered?
S1 just grew too large already and there was a big change in the story that gave an opportunity for the developer to leave the content from S1 behind and slim down the next season back to a manageable size.
Confusing might be the order Incetongames published these parts, but it's been asked and answered multiple times already, so you could just try to make use of the thread search feature.
Also, since the new S1E10 is a complete rewrite of the S1, I doubt that any of the old saves from previous S1 parts will work.
I am planning to start it from the beginning myself to see everything that has been added to it or improved compared to the original release and first playthrough.
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BTW, what really confuses me personally right now are the likes I've received today for the bugfix I made for S2E1
- Are these guys playing S2E1 now and not the new S1E10, so my bugfix helped them?
- Did they apply that bugfix to S1E10, although its link is hidden inside the spoiler of the S2E1 in the OP, clearly marked as a bugfix for S2E1 only and not to be suitable for any other version in my post?
- Is that bugfix actually usable for S1E10 to help with the same issue with angle and bonus image screens without breaking anything else? (It is possible, I guess)
- Do they need a bugfix for S1E10, so they are giving me a "call" to get busy and make one?
- Are they just saying thanks for it now because they forgot to do this when they played S2E1?
Thanks guys, but I have no idea what you mean... ¯\_(ツ)_/¯
Anyway, I'm a bit busy with some other stuff and haven't had a chance to look at the new S1E10 and probably won't have time for it for several days. Sorry, if you guys actually expected something from me for the new S1E10. I can't make it this fast.