Because ChaCha20's block migh not even fit into memory on platforms that is Speck/Simon meant for. The question should be: Why not use XTEA or RC5/6.

And the answer (ignoring the NSA and it's opaqueness) is that Speck (in its reference implementation form) is slightly faster than both XTEA and RC5 and can be trivially implemented such that it is twice as fast and also uses 64b words in 128b block size variants which on typical 64b platforms leads to another essentially free 2x speedup. (Not that it makes sense to use Speck for bulk encryption on typical 64b CPU)

On the other hand team related to DJB published algorithm called Gimli (https://gimli.cr.yp.to/) whis is essentially narrower ChaCha 20 transformation with intentionally slow diffusion as to fit onto registers of register-constrained CPUs (like Thumb) that is intended as Keccak-style sponge permutation. On the other hand in my opinion authors' paper and slides also have similar issue as what part of the original article criticizes NSA for, the security arguments for Gimli are considerably better than NSA's arguably non-existent ones but then the bigest issue I intuitively have with Speck is slow (and somewhat one-way) diffussion of the key schedule and Gimli's 4-round function is somewhat similar or even worse in this manner (although it does not have the "onewayness", for lack of a better word).

See ebiggers' comment above - ChaCha20 isn't on its own suitable for disk encryption. We need something that is a drop-in replacement for AES to use in XTS mode.