15 MB/s is typical for good SD cards (e.g. Class 6), which are limited by design to 20-25 MB/s anyway (UHS-1 SDHC will be faster, but is still rare today). High-end SSDs can be much faster for a number of reasons:
* SATA is a much faster interface than SD/MMC
* NCQ and write caching allows optimizing the accesses by reordering and batching NAND flash accesses
* Using SLC NAND instead of MLC improves raw accesses
* Using multiple NAND chips in parallel gives a better combined throughput
* Expensive microcontrollers on the drive can use smarter algorithms
All of these cost money, so you don't find them on the low end drives that I analyzed.