Go on then, I’ll bite (this is a troll right?)
Because carbon is better (for the pro)?
You might want a bit of spring/zing/give/insert ti/steel marketing bolloks here in your frame, he/she justs wants the most efficient frame he/she can get.
You might not mind a 1500g frame, he’d rather not spend the winter getting down to 3% body fat then have to drag another 1.5lb of frame up 3 alps a day for 3 weeks.
You might want a bike for life (ignoring the 101 cracked ti frames on here at the moment), he just wants it to last a season*, if it is still going for the spring classics its a bonus.
*the average rider probably isnt doing 100mile training rides every day so carbon frames probably last the same ‘lifetime’ as Ti anyway.
You might not want/need an aerodynamic frame, he’d quite like to be in the breakaway, get some TV time and make sponsors happy.