The reason is simple. Michal Szpak has a big (and partially hateful) fanbase. These fanboys are ready to commit terrifying things in the name of their idol - just like Directioners and Beliebers. That's also the main reason why he won KE.
Well, speaking of KE, TVP never shown up an amount of votes, we got only precentage. I remember that years ago during final of Eurovision Song Contest the first place got around 30.000 votes. This is not that much + remember that KE had really average viewership, something about 2.3 Million (and for example performance of Monika Kuszyńska in the final ESC15 was watched by almost 6 Million people in here). As I said in this topic before, one televote costed 3.50PLN and regular SMS costs like 0.09-0.19PLN (depends on your airtime provider). I know that TVP wanted to earn some money, but they could choose cheaper option of premium-sms, like 1.00PLN or so. So when a lot of people watch, but really few of them vote, this is not that hard to win. I am not saying that someone was cheating - it was more like fanbaeses fight, because Margaret's and Edyta's ones were also very determinated to win and for example Natalia Szroeder who is also very popular in here and her song is on the top of the charts, but I couldn't see any pressure by her, got only 4% as I remember. I think that Michał got also some extra votes from people who just liked his voice and from the people who think that Eurovision is not a song contest and it's good to send someone controversial (because he is that) - I remember that after Austria's 2014 victory there were a lot of memes, that 2015 is the time for Szpak and a lot of Poles still thinks that ESC is a competition for weirdos which is really not fair, because I can find in here some good songs (every year).
Fanbase of Michał Szpak is for me like a social phenomenon, because I wasn't expected that he has that big number of fans - his FB profile or results on the album/airplay charts suggest the opposite way and seriously they are really active and sometimes hard-core. I had no idea that those people can affect so many YouTube videos, polls etc, but I totally understand that they support him (because they like him) and I haven't notice that they hate another ESC participants (although, I still can see comments and weepings that Michal Szpak is a target of hate in Poland which is not true, but they post mean things about another singers from KE easily). Comparing them to Directioners, well... Szpakoners are a little bit older, I guess.
My point is - I can imagine that all those Szpak's fans can overwhelm the Internet by millions of coments that Eurovision is not a music contest for such talented and charming people like Michał in case that he's lost in the semis or got bottom place in the final, but I hope this will not happen.
As a viewer from Poland I am glad that he has many views on the YouTube and a lot of positive comments in there, but it will be veryfied in May.
Seriously, I am curious and confused at the same time.