It is my understanding that the design flaws of WEP were well known from its beginning.
WEP on Wikipedia states, was introduced in IEEE 802.11 in 1997, and it was based on the stream cipher RC4.
There has been quite a lot of research to break WEP and its stream cipher RC4, but that's spread out over the last 20 years. Most notably for breaking WEP was the attack from Fluhrer, Mantin and Shamir in 2001. And earlier in 2001, this work was presented at a workshop and a conference (thanks @poncho for this reference).
For RC4, it was public knowledge that it is distinguishable from random, as pointed out by @SqueamishOssifrage in this source. Today that would surely disqualify the streamcipher to be considered secure, but mindsets change over time and I don't know if that was acceptable or not. E.g. the linked source states: "If your application can't stand a bias of 1 part per million, then
don't use RC4." Also, we do not know whether the people in charge of WEP were aware of this or not.
So no, your statement is wrong. Just because the flaws are well known today does not mean they were back then. If you're questioning decisions, you need to adapt the point of view from the time of that decision.
In WEP’s design process, why were no people with sufficient “expertise in cryptographic protocol design” involved in the creation of a cryptographic protocol intended for broad adoption?
This can't be answered properly, unless you ask someone who actually was involved in that process. The only suggestion I have would be to contact the IEEE 802.11 working group and ask them.