No question of that, they knew I was waiting on a new build that’s due to be ready sometime in the summer but no concrete date at the mo,Look at this from the buyers perspective. They don't want to sign a binding agreement and then risk you stringing them along for years risking that the market or their needs change significantly. Remember, if they are borrowing, they'd need to re-apply for mortgage approval if this drags on beyond their current approval period. They may be aware of potential changes to their circumstances and under pressure to close in advance.
Purchasers coming from rented accommodation will want to stop paying what they perceive as dead money ASAP and may be under pressure to vacate. Purchasers trading up or down may have prospective buyers lined up who are putting them under pressure to commit to a date.
The purchaser's solicitor is just doing their job here. No solicitor worth their salt will allow their client sign a completely open ended contract.