They have a big commission for transactions in some countries , and some people ended up with their account blocked and having no wayu to get their money out.
I am just a user, not a merchant so I am not sure what the issues would be from that side of the transactions, I personally avoid keeping too much money in PayPal just in case they somehow block my account for some bullshit reason.
Edit: I would prefer not to go off topic, unless is related with why some big companies refuse to use PayPal and others do use it. As an example I could not buy audio books from amazon so my money went to a different company that accepted PayPal.
If a company implements paypal then people who only have paypal might pay them money that they otherwise wouldn't. However, people who might have otherwise payed via another means may also now use paypal. If paypal has higher fees this could well result in less money in revenue.
Also, maintaining each payment gateway has an implementation and maintenance cost. Add it all up, and assuming that the number of potential customers like you that only have paypal is small, and it's easy to see why companies may choose not to implement it.
Who knows. Every company is different, has different customer bases, different histories, different technologies. Maybe they signed a deal with paypal where they get lower fees, maybe they integrated it in a time when it made more sense then now (e.g. before apple pay and google pay were so prevalent), maybe they outsource their payment to a processor that just supports it.
I don't know the details of any of these places, I was just giving a reason why a company may not implement a particular payment processor. It's based on a balance of factors, not just a simple "support X get more paying customers."
I am just a user, not a merchant so I am not sure what the issues would be from that side of the transactions, I personally avoid keeping too much money in PayPal just in case they somehow block my account for some bullshit reason.
Edit: I would prefer not to go off topic, unless is related with why some big companies refuse to use PayPal and others do use it. As an example I could not buy audio books from amazon so my money went to a different company that accepted PayPal.