It's not quite that blatant, but there's some truth to that sentiment... they were under a big crunch before the franchise essentially folded (on the board game front... some reprinted 2nd edition boxes appeared in other territories and in the UK there was another HQ novel in 1993, then the Amiga computer game HeroQuest II: Legacy of Sorasil in '94, to be fair). So they probably figured get a quick cash in before it was too late.
Despite being more difficult, BQP was finished first. It was a big meaty campaign, designed to kick the butts of players who found the game system "too easy" but they (like many makers of sequels to popular but "mastered" games) over-corrected to be sure... add that to the crunch factor above to explain the issues. Fast forward to the modern age, AH was still in "don't change it" mode with regard to re-releases. Witness the corrections in the box to the Elf pack released after, and the updates and revisions being introduced into ATOH in a few months!
The old drafts (thanks lucapaschi) indicate other ideas were being discussed, some of which could have softened the difficulty. Otherwise they were relying on hardcore players to fix the quests for them (I pity any first time young players that actually tried to enjoy those "solo" quests as truly for beginners!). In the modern age "Into the Northlands" and its addendum with the playtesting, etc. is a nice starting point for revisions that most players will want to consider. But some of the old drafts provide some ideas too...
For instance the Yeti hug escape mechanic was in the drafts but ignored in the actual release (1 BP damage per turn after being hugged the first time the Yeti damages the hero instead of 2... each turn hero rolls 2 combat dice... 1 white shield is no damage that turn, 2 white shields means immediate escape... so you see not as forgiving as the new mechanic in the HQ companion App which is 2 red dice and any 5 or 6 rolled is an escape while still getting 2 BP damage otherwise OR you can just click the button to get an immediate escape if you toggled escapes in settings, but still!). Also, in the draft notes a hugged hero (and monster) would still defend with one less each (so the Yeti rolled 2 defense). They intended it to be a coop thing, where you save your buddy from the monster, but the escape mechanic was essential for the three solo adventures! The presence of unmarked treasure chests also shows that the never-found artifacts have a place in the game (more of these unmarked chests in Mage of the Mirror). Into the Northlands gives the suggestion to add animal companions (to save you from the Yeti) and/or simply nerfing the Yeti so they don't hug anyone if there is only one heroic character in the quest at the time.
They were toying with changing the search mechanic, to say you would need to move up to treasure chests to open them (but via the fact they didn't follow through and the recent Stephen Baker interview show that this was never the intention any the game up to this point, and yet players still house rule it and even quest designers occasionally forget and put traps next to treasure chests!).
Mage of the Mirror was actually harder, originally. Yes, you could buy from two different shops (one that sold potions that were half price but had 50% chance of failure via die roll), but Ogres were introduced in the very first quests (mostly erased or replaced with Fimir). There were also more numerous Giant Wolves. There was to be a "blind jump" and "double jump" mechanic... you could choose to jump ANY square (even before searching) meaning you could leap through doorways and not fall into the trap every single time. I imagine that rule would have been applied to BQP as well above. The double jump meant you lept over two squares but had to roll a white shield on one die (failure meant you just landed on the first square and proceed like normal, but you could always attempt to jump the next square, etc. etc.). The other mechanic left on the cutting room floor was to allow a room with multiple pieces of furniture to have each furniture piece searched individually (and perhaps another search of the "rest" of the room too? unsure). It really was only intended for the solo quests (would have slowed things way down if these rooms allowed multiple searches per hero! and the treasure deck would have been depleted of good cards much faster potentially) presumably to build up the suspense.
You can see traces of the removal (or non-implementation) of these mechanics in the first Elf quest, the second Barbarian quest, and any time you see two trap tiles in a doorway.
On the bright side you could look at it this way... even though each of these packs purports to have 10 quests (3 solo, 7 group) it's really more like they gave you 17 group quests each. Someone who really wanted to overhaul the whole thing would probably either start deleting monsters and traps like you say, or split the big quests into two each.