Because the W500 doesn't support more than 8GB ram and uses the old Sata 3Gb/s interface which I believe would be a bottleneck with the latest SSD's.
Personally I'd go with a new Thinkpad. MBPros are nice but the new ones are completely non-upgradable which is a real deal breaker. I've used a T500 for a little over 4 years now with nothing but more RAM, a new SSD, and a storage bay HDD.