I never understood the firing of Slocum, but it was probably time I guess.
The main reason was he was being passed by Mack Brown and Bob Stoops.
A couple of things happened at the end of Slocum's tenure as coach that probably skewed his record. We fell behind in the recruiting wars, we didn't invest in facilities early enough and by the time we started both OU and UT had top notch facilities to attract much better recruits.
The big issue with Slocum, in my opinion, is not how much he lost to OU and UT, but how much he lost to TxTech, Mizzou, Kansas State, and the rest. In reality, nobody in the Big XII could have kept up with Stoops and OU. They were the Blue Blood at the top of their game. Winning 25% of the games against them would have been seen as progress. I think Slocum won 1/4, with close losses in 2000 and blowouts in '99, 01. We won the 2002 game, defeated #1 OU at Kyle. I bet Mack didn't fare much better, but maybe 30-40%W and Texas was doing very well at that time.
The mistake A&M made at the time was letting the 2nd and 3rd best recruits get away. We spent so much time recruiting the same players as OU/Tex that we didn't end up signing that some of the best recruits of the era went to TT, KSU, oSu, and TCU. We could have easily fielded a top 10 team of the 2000's if we simply focused on the "best of the rest". Adrian Peterson is a good example of that. It was reported at the time that it was between OU and A&M (I think it was around 2002 timeframe). I don't think I can tell you who the RB was for A&M from that era, but I'd be certain that some of the ones that played for the other teams were probably a lot better. The only one that stands out to me from that timeframe was Jovorskie Lane, and he was a FB that played RB.
Anyways, a lot of the frustration around Slocum was based on the fact that he was in his 12th year, and not only were we losing to OU/Texas but now we were losing regularly to TT, KSU, Mizzou, and oSu. I don't disagree with the change, it was just who we hired that was the biggest mistake. Fran was an abject disaster.