No. Viruses can infect any commonly used bootable media.
The spread of this type of viruses became thwarted (in addition to better protection mechanisms) because of a change in media usage patterns.
Floppy disks in their golden age were used as a medium for live OS (booting and running), software distribution and for data exchange.
Nowadays you use:
- non-removable hard disks
- write-once read-many optical media (like CDs, DVDs, DVD-Rs)
- USB drives
Although you can still boot an OS from either of this media, they perform different functions. This alone (not by enforcement, but by convenience) made the propagation of boot sector viruses rare, despite still being technically possible.
For a boot sector virus to spread you must insert a clean bootable media into an infected machine and later use the media to boot another machine.
This is extremely rare case for HDDs (non-USB) as you don't use them for software distribution or data exchange.
In case of optical media, they can be used to spread viruses, but they don't allow for cascading spread of a boot-sector viruses like with floppies.
USB usage pattern makes them most vulnerable of the three, but still it is relatively rare case to boot from USB. And USB media require special attention anyway, so additional procedural measures are taken.