It may also have been a design of CA to give some change in the gameplay...

Let me explain...

Fighting open field battles, it seems logical that a charge of cavalry should bowl over everything that stands in its way until impact then it would e subject to flanking or sword fighting, not to its benefit...

On the other part, in siege battles, infantry rules and makes cavalry useless (and mortally so)...

Thus it allows two different styles of gameplay.

Lastly, thinking on it, it sounds logical to me that no unit except those with pikes or at least spears should withstand a formed cavalry charge.