Not sure of all the reasons since I wasn't involved in the rule voting but my guesses include some of the following ideas:
1. Teamwork in Cat P-1-2 races can be critical. If a racer is "mislabeled" it can really throw things off.
2. In 3s-5s it's less critical (although it still has an effect, mostly in the 3s).
3. A "new" team may not have gear. "New" meaning a few guys who got together to make their training ride group into a team. Not likely to be a 2, more like a 4 or a 5.
4. Some teams (haven't seen as many recently) actually have contracts. To get a 2 is one thing - to retain a 2 is another.

Basically a strong 3 can pull back a break all by himself that is pretty far up the road. Ditto a 4 or a 5. But a 2, that would be tough. Teamwork becomes a bit more important when one or two racers can no longer affect the outcome of a race, and in the 2s I think the racers are more evenly matched in terms of fitness.

In addition the matching kits would be an indication of commitment to the sport. A Cat 5 may not feel a commitment to the sport enough to buy a team kit even if they belong to a local team (they might have joined, for example, to get a discount on a bike).

cdr