Why did "My Two Dads" struggle while "Full House" succeeded?

In 1987, two sitcoms with relatively similar premises, one on NBC, the other on ABC, premiered. While one limped along for three years, the other became a hit. Yet, as is often the case in TV history, the one that became the hit wasn't necessarily the better of the two shows.

To put things into proper perspective, NBC's My Two Dads, which premiered on September 20 of that year, featured Paul Reiser and Greg Evigan as two men who have to live together to raise Nicole (Staci Keanan), the daughter of the now-dead woman they both slept with at the behest of a judge (Florence Stanley).

Despite the improbability of the premise and the overall mediocrity of its execution, powerful competition on CBS from the 9th ranked Murder, She Wrote and its erratic scheduling on top of that, it still managed to rank 21st place for the year, even outranking Day By Day, the show it alternated the coveted post-Family Ties time slot with but could only muster 43rd place; Family Ties came in 17th. But Day By Day was from the same producers as Family Ties, which gave it the advantage at renewal time; it made the fall schedule while My Two Dads had to wait until mid-season to come back.

It did manage to make the 1989-1990 fall schedule while Day By Day ended up being cancelled the same year its more celebrated lead-in bowed out voluntarily. But by mid-season, it kept getting moved around between Wednesday and Monday, and NBC decided it was a lost cause, so they cancelled it.

Meanwhile, in its first season, Full House (the premise wasn't particularly outlandish in and of itself—a widowed dad with three daughters asks their two uncles to come live with him in San Francisco and help him raise them) ranked three whole ratings points below Webster, the show it replaced that ran for another two years in first-run syndication anyway despite still pulling better ratings in its last ABC season than Full House's first.