Nintendo first demonstrated the Super Famicom hardware in November 1988: https://www.chrismcovell.com/secret/SFC_1988Q4.html . If you look at the hardware specifications listed, the only thing they changed between then and when it came out in 1990 is increasing the work RAM from 8 KB to 128 KB. I imagine they sat on the console for so long because the Famicom was still selling and they wanted to take the time to develop a good library of launch titles.