There are two matters here - one concerns whether Matt 23:14 is actually part of holy Scripture or not - NA27, NA28, USB4, USB5, W&H, etc, all omit this verse. However, let us assume, for the moment that the verse is valid and answer the other matter - why greater condemnation?
Now, whether Matt 23:14 is valid or not, there are other places in the Bible that have a similar idea:
- Mark 12:40 - "They defraud widows of their houses, and for a show make lengthy prayers. These men will receive greater condemnation.”
- Luke 20:47 - "They defraud widows of their houses, and for a show make lengthy prayers. These men will receive greater condemnation.”
- James 3:1 - Not many of you should become teachers, my brothers, because you know that we who teach will be judged more strictly [= greater condemnation, KJV].
In each case the phrase in question is either περισσότερον κρίμα (Mark 12:40, Luke 20:47), or, μεῖζον κρίμα (James 3:1).
Before proceeding further, we should remind ourselves of the difference between two nouns that are often simplistically translated as "judgement", namely, κρίμα (krima) and κρίσις (krisis). According the BDAG, the distinction is important:
- κρίμα (krima) is the final decision when the legal process of trail is complete, ie, verdict, condemnation or sentence.
- κρίσις (krisis) is the legal process of judgement, ie, the trial process by which one gathers data and evidence before arriving at a decision.
In the above texts, we are discussing the first meaning - the verdict or sentence received the the guilty against whom Jesus pronounced the "woes", the "scribes and Pharisees, hypocrites". That is, Jesus is saying that these sinners (in Jesus' eyes) will receive a harsher sentence in the judgement than others.
The reason is not hard to find. Their sin was much greater precisely because of their privileged position - they had greater knowledge, they could read the holy Scriptures (unlike many), and they were the appointed teachers in Israel.
Note again what James says: "Not many of you should become teachers, my brothers, because you know that we who teach will be judged more strictly".
Jesus said something very similar in Luke 12:47, 48:
That servant who knows his master’s will but does not get ready or
follow his instructions will be beaten with many blows. But the one
who unknowingly does things worthy of punishment will be beaten with
few blows. From everyone who has been given much, much will be
required; and from him who has been entrusted with much, even more
will be demanded.
In the case of these pompous, self-sufficient and overtly (but falsely) pious scribes and pharisees, their sin is made much worse because they effectively steal from the poor including widows, etc. The Expositors Commentary correctly observes (on Mark 12:40)
In it and the two preceding we have a very slight yet vivid picture of
Pharisaic piety in its vanity, avarice, and hypocrisy.
I agree.