Because there are already Real Madrid and Liverpool going after him. There is no way Newcastle can race with those teams for him. Of course I know that Newcastle would be ready to pay whatever he wants as they have a huge budget. But money isn't enough by itself in this. Newcastle don't have any achievement yet. Finishing the Premier League in top 4 isn't enough for Bellingham to choose them. He would like to sign for a team that have a much stronger squad. He would prefer a successful team as well.
However, he may move to Newcastle under the guise of a promising club. They started to strengthen very well and started to win as well. Everyone understands that it is almost impossible to strengthen once and try to beat everyone. Newcastle are still building their squad and they have excellent results and can achieve a lot in the future, so choosing this club is a prospect for the future
Maybe, I don't know if FFP will let them sign him for a lot of money, but a lot of players go for the money and he might not be an exception. In general I meant that the team has had and will continue to have reinforcements, but if FFP allows them to sign only one player the club owners themselves will give up the idea, because it makes more sense to sign several players rather than one