For a few reasons, but most simply because more states voted. In the 1789 election New York did not send electors because they couldn't decide how to appoint them (they debated common vote vs legislative vote/appointment amd failed to decide - most states in early America allowed the legislature to vote for president, not the people directly) before the deadline to do so. Rhode Island and North Carolina had not ratified the Constitution so they were likewise unable to participate. In 1789:

Massa-Maine (Massachusetts but also including modern Maine) had 10 votes.

New Hampshire had 5 votes.

Connecticut had 7 votes.

Pennsylvania had 10 votes.

New Jersey had 6 votes.

Delaware had 3 votes.

Maryland had 6 votes.

Virginia had 10 votes.

South Carolina had 7 votes.

Georgia had 5 votes.

Total = 69 voters, times two votes for each delegate. Waahington recieved 69 votes, Adams recieved 34, the rest recieved 35, cumulatively.

1792 election:

Massa-Maine had grown 6 to 16 votes.

NH grew 1 to 6 votes.

Vermont joined the club of states and brought 3 votes with her.

Rhode Island also joined the club, bringing 4.

New York got her act together and brought her 12 votes.

CT grew 2 to 9 votes.

Pennsylvania grew 5 to 15 votes.

New Jersey grew 1 to 7 votes.

Delaware remained at 3 votes (even today little Ol' Delaware still has only 3 EC votes).

Maryland grew 2 to 8 votes.

Virginia grew 11, the largest gain of any state, to come in with 21 total votes.

Kentucky was added and brought in 4 new votes with her.

North Carolina also came in with 12 new votes.

South Carolina grew 1 to 8 votes.

Poor Georgia lost a vote, dropping 1 to 4 votes.

Total = 132 voters, times two for each delegate.

New additions = 35 votes

Growth additions = 28 votes

Results: Washington again cleared house with 132 votes (of a possible max of 132 votes). Adams again solidly secured second with 77 votes. George Clinton (not the phunk master, though) scored 50 votes, and my boy Jefferson pulled the 4 from Kentucky despite his desire not to be involved at all. The final lone vote went to Aaron Burr.

The House progressed from 59 seats in the First Congress (1788) to 67 in the Second (1790) to 105 in the Third (1792), all from growth, which is what accounts for the other votes (however the math isn't that simple as some votes are "double counted" in this methodology - for instance the addition of new states is included in the above congressional seats in addition to growth of existing states in representation).

Hope that makes sense and answers your question. Feel free to ask any follow ups if it doesn't.