Why can't ransomware practically use RSA to encrypt all files?
RSA is really very slow compared with symmetric ciphers. 
You can check this yourself running e.g. openssl benchmark:

openssl speed rsa


On my machine (and with openssl 1.1.1a) I get 

Doing 2048 bits public rsa's for 10s: 330640 2048 bits public RSA's in 10.00s


so we can do ~33k encryptions per second and the size of the encrypted block is less than the modulus, so less than 256 bytes. If the attacker wanted to encrypt 1GB file, it would take about 2 minutes. 
On the other hand, 

openssl speed aes


shows that AES-CBC can encrypt ~300MB per second

type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes  16384 bytes
aes-128 cbc      94234.08k   136019.99k   137980.25k   304759.81k   304166.23k   315654.14k


so the same 1GB file would be encrypted in 3 seconds. 

For the attackers, speed is important, since the faster the files are encrypted, the less likely the process will be interrupted.