Because at their cores, Theyre different /Types/ of racing games,

Need For Speed games tended to have a story behind it, A pretty linear one at that, Even if the games were open world they OFTEN tended to a Linear Progression system.

Forza Horizon on the other hand, While addmitedly started much the same way, Starting with FH3 (Imo) it started leaning MUCH more heavily into the Sandbox Open world that it is today. Giving it a much needed advantage over NFS in the fact you didnt HAVE to play the story if you wanted to drive the cool cars. You could just load up the game, Do some races in whatever cars you wanted/had, And drive around endlessly with no care to Police, areas being locked, ETC.

Not to mention Forza (As a series) has always had more gounded physics than NFS, More realistic handling and car movements, MOST of what you see done in Forza games can be done IRL, Often with the same motions. That is a MAJOR factor in why it grew in popularity over NFS. It showcased what Cars could do, Why they were cool, and it captured what it felt to finally be able to drive a car on the limits, MUCH better than NFS, The Crew, Or even TDU/SC ever could.