Simple Electromagnetic Analysis vs symmetric cryptography
SEMA is effective against a naive implementation (e.g. using binary square and multiply) of raising to a secret value in some group. Raising to a secret value in some group is used in common asymmetric cryptography techniques, including based on hardness of integer factorization (RSA) or the discrete logarithm (DSA, ECDSA..). That's why SEMA happens to be effective against naive implementations of common asymmetric cryptography.

It would be wrong to say that only asymmetric algorithms are vulnerable to SEMA. For example, the Pohlig-Hellman exponentiation cipher is a symmetric encryption algorithm where encryption goes $x\mapsto x^k\bmod p$ for public prime $p$ with $(p-1)/2$ prime, and $k$ a secret exponent. A naive implementation would be vulnerable to SEMA just like a naive implementation of RSA is. And it is all too frequent that a very naive implementation of DES is vulnerable to SPA/SEMA.

It would also be wrong to say that all asymmetric algorithms are vulnerable to SEMA. Careful implementation of raising to a secret value can be made secure (using the Montgomery ladder or other techniques, including blinding). And some asymmetric algorithms (e.g. Lamport signature) do not raise to a secret value, and can be made immune to SEMA with careful coding and control of the underlying hardware.