Many older "vintage" VSTs were never updated to 64-bit. Using the 32-bit version of FL Studio allows these plugins to run without the CPU hit of "bridging."
Version 20.8.4 is the last release that can run on a 32-bit Windows operating system. Plugin Bridging: Fl Studio 20 32 Bit Download Windows 7
Many "vintage" VSTs only exist in 32-bit format. Many older "vintage" VSTs were never updated to 64-bit