May ToC better than Feb ToC...overall better weather and the field is more competitive further into the racing calendar.
Not all teams choose to compete in the Giro, and not all teams are invited to the Giro. If you're focused on the TdF, then those riders and teams would rather race warm-up events, which is what the ToC is (Dauphine Libere and Tour de Suisse are other shorter stage races on the UCI calendar that are used to build up to the TdF). There's always a second-level event or two opposite the grand tours that quality teams ride in.
May ToC fits into the secondary role nicely, starting at least a week after the Tour of Romandie and ending at least a week before the Tour de Suisse...and AEG has made friends with ASO (for better or worse) so there's some benefit there in planning ahead for scheduling so it avoids confilct with the other 2nd tier events. ASO's involvement also gives some greater weight to the ToC....remember that since Radioshack wasn't going to the Giro, they weren't even invited to the other races put on by that organization (Milan-San Remo and Tirreno-Adriatico to name a few).
Sure the racing at the Giro is better...its a Grand Tour in a cycling country, and each one tries to up the ante over the others...that's why Sunday's stage used a road with pitches over 20% and the route profile for this year has been described as "brutal" at best with the second half pretty much eliminating the sprinters who didn't crash into the road furniture in the Netherlands.