Are retail IPO allocations mostly the deals institutions don’t want?

I’ve been thinking about IPO offerings and how shares are allocated. Is it reasonable to assume that the most attractive IPOs are largely bought up by institutional investors before the general public gets much access? In other words, when retail investors are offered IPO shares, is that sometimes a warning sign that institutional demand wasn’t strong enough to absorb the offering? Or is that too simplistic, and are there other reasons retail investors might receive allocations even in a desirab