As rightly pointed in the question, 0*+length padding would work just as well as 10*+length padding, with the benefit of simplicity and requiring one less block in some cases, like SHA-256 for a message of 56 bytes.
I guess SHA-2 does this because SHA-1 did, and before that SHA, MD5, and Ronald Rivest's MD4 (1990), which is the earliest reference that I can point, and already had:
Step 1. Append padding bits
The message is "padded" (extended) so that its length (in bits)
is congruent to 448, modulo 512. That is, the message is
extended so that it is just 64 bits shy of being a multiple of
512 bits long. Padding is always performed, even if the length
of the message is already congruent to 448, modulo 512 (in
which case 512 bits of padding are added).
Padding is performed as follows: a single "1" bit is appended
to the message, and then enough zero bits are appended so that
the length in bits of the padded message becomes congruent to
448, modulo 512.
Step 2. Append length
A 64-bit representation of b (the length of the message before
the padding bits were added) is appended to the result of the
previous step. In the unlikely event that b is greater than
2^64, then only the low-order 64 bits of b are used. (These
bits are appended as two 32-bit words and appended low-order
word first in accordance with the previous conventions.)
Perhaps earlier block-cipher based MACs used 10* padding without length strengthening, where 10* prevents some trivial attacks 0* allows. In particular, 10* padding was used for variants of the so-called "CBC-MAC", which became ISO/IEC 9797-1 MAC Algorithm 1 padding method 2 (approved 1999). It would then be plausible that the 10* padding was kept, and length-strengthening as suggested by Merkle added, because 10* padding alone would not be as safe.
Alfred J. Menezes, Paul C. van Oorschot and Scott A. Vanstone's Handbook of Applied Cryptography, in section 9, mentions both padding with 0* (algorithm 9.29) and 10* (algorithm 9.30) and note:
The padding method of Algorithms 9.29 and 9.30 originated from ISO/IEC 10118-4 “Information technology – Security techniques – Hash-functions – Part 4: Hash-functions using modular arithmetic”, draft (CD), 1996.
but I doubt this is the true origin.