Cost. SLC is limited in capacity. 60GB SSDs have no market. 1TB (MLC, or eMLC) SSDs do. Vendors have started advertising the write endurance of SSDs as DWD (disk writes / day). The advertised values are 1 to 3 DWD. So a 1TB disk is guaranteed by the vendor to meet the life expectancy (usually 5 years) with 1 to 3 TB / day written to that disk. Actual endurance is usually better than advertised.
Cost. SLC is limited in capacity. 60GB SSDs have no market. 1TB (MLC, or eMLC) SSDs do. Vendors have started advertising the write endurance of SSDs as DWD (disk writes / day). The advertised values are 1 to 3 DWD. So a 1TB disk is guaranteed by the vendor to meet the life expectancy (usually 5 years) with 1 to 3 TB / day written to that disk. Actual endurance is usually better than advertised.