Discrete logarithm problem particularly hard for Schnorr groups?
I do not know an exact citable source.

However the rationale is roughly the following:

The Decisional Diffie-Hellman Problem (DDHP) is not hard in $\mathbb{Z}_p^*$ since the Legendre symbol leaks the parity of the discrete log (DLOG) [1 section 5.1.2, 2]. You can use this fact to build an efficient algorithm to distinguish between Diffie-Hellman and non Diffie Hellman tuples [3 section 6.6]. 
Another reason is the existence of the Pohlig - Hellman Algorithm [1].

Note that the semantic security of the ElGamal cryptosystem rests on the DDHP problem, which is believed to be easier than the DLOG problem.

References:

[1]Cryptography: Theory and Practice by Douglas Stinson

[2] Finding $x$&#39;s parity in the discrete log problem

[3] https://crypto.di.uoa.gr/class/Kryptographia/Semeioseis_files/Cryptograph_Primitives_and_Protocols.pdf