Because there is a trend of not trusting experts or science
There is a rise of the conspiracy theory
The Internet has led to a rise in people doing their own 'research' which is basically googling and then believing the results and not thinking about the source or quality of the study.
I saw some research about how children who had one type of vaccine in NZ were then more ill. It was a link on social media and had loads of people saying how they were now going to not vaccinate and how finally there was proof etc. I looked into the study. It was about 100 people. Not enough to be valid. It was sponsored by a company that pushes alternative remedies so will be biased. It was based on self reporting and was obviously of more interest to people who don't vaccinate as there was a much higher proportion of ubvaxxinated children than for the general population. Questions were things like 'how often do you take your children to the doctors / give antibiotics' etc which of course are going to be lower for people who don't believe in vaccinations are going to be much more likely to avoid 'western' medicine. So the whole study was a sham. But no one was interested in that just the headline that vaccines make children more ill as it validated what they already thought.
Other reasons are that the diseases we vaccinate against are usually mainly obsolete now so people just rely on the herd immunity of others (they know the chances of actually catching these diseases are low but don't acknowledge this is due to vaccinations)
They haven't seen the damage that these diseases can do. They think measles is just a spotty cold and don't actually realise it can kill and has quite a high percentage change (1 in 15) of lifelong complications including deafness and blindness
Also there is a load of half truths and mis information out there. For example that the flu vaccine can give you flu (it can't), that illesses are actually spread through the vaccine shedding (they don't as it's such a weakened version of the vaccine) and they are full of formaldehyde to make you ill (they do contain this but in miniscule amounts which are actually a tiny percent of the amounts anyone is exposed to daily and any excess is just secreted in urine)
Lastly you have clueless celebrities saying things like it's better to space vaccines out as having them all at once will overload the immune system. Despite studies showing that vaccines have less strain on the immune system than babies and toddlers are exposed to every day from doing baby and toddler things like picking up old food from the floor and licking shoes etc