Primarily because of the orbital paths.    Mercury is closer to the Sun and therefore its orbit is much smaller than Venus.   Consequently, Mercury moves faster than Venus and completes an orbit in much less time.  Mercury needs about 88 days to revolve around the Sun.  Venus requires 225 days.     These times relate directly to the planets' "synodic periods."  A synodic period is the time a planet requires to return to the same position relative to Earth.  Mercury's synodic period is 116 days, which is the time that separates successive inferior conjunctions.  Venus' synodic period is 584 days.    Mercury will pass through inferior conjunction about three times a year, but more than a year and a half separates each inferior conjunction of Venus.    Remember that a transit only occurs when the planet is at a node the same time it moves through inferior conjunction.  Mercury is in inferior conjunction more often than Venus and its transits will be more frequent.

We'll start by listing all 21st century transits of Mercury. 

  2003 May 07   
  2006 Nov 08   
  2016 May 09    
  2019 Nov 11
  2032 Nov 13 
  2039 Nov 07
  2049 May 07 
  2052 Nov 09   
  2062 May 10  
  2065 Nov 11   
  2078 Nov 14   
  2085 Nov 07  
  2095 May 08 
  2098 Nov 10

Five of the 21st century transits of Mercury happen in May.  The other nine occur in November.    So, yes, November transits are about twice as numerous as May transits. To explain this difference, we need to discuss the concept of "Transit series."      

We recall that transits only occur when an inferior planet is at a node during inferior conjunction. A node is an intersection point between two orbits.  Two planetary orbits will actually have two nodes, ascending and descending.   When Mercury crosses the "ascending node," it is moving "north" of Earth's orbit.   When Mercury crosses the descending nodes," it is moving "south" of Earth's orbit.     Transits can and will occur at both nodes.     May transits occur when Mercury is at the descending node.   November transits happen at the ascending node.   A transit series consists of a sequence of transits happening at either node.     

For example, Mercury transit series 8 consists of a sequence of November transits.  The first occurred on November 2, 1776.    That transit just "grazed" the southern tip of the Sun.    The next transit in that series happened on November 5, 1822.   During that transit, Mercury moved along a "higher" chord, though it was still "low."  The following Series 8 transit was on November 6, 1868.  Mercury's transit path on that date was even higher than the November 5, 1822 path.     Since November transits occur when Mercury is at ascending node, each successive transit within a given series will migrate "northward" until the highest chord is reached, at which point the series ends.    Mercury transit series 8 concludes on November 23, 2604. 

[Perhaps a couple graphics will help you.   See them on the Daily Astronomer web-page: www,usm.maine.edu/planet/da-7-december-2015 ] 

Transit series 9 consists of May transits.  The first such transit occurred on May 6, 1957.   This transit path sliced across the northern tip of the Sun.    The next transit in series 9 happened on May 7, 2003.    Since May transits occur at the descending node, this transit path was "south" of the May 6, 1957 chord.  The next series 9 transit -May 7, 2049- will be south of the May 7, 2003 transit chord.  (Note:  The transit on May 9, 2016 is part of series 7.)     The Transit series 9 ends on May 16, 2371. This transit path will cut across a very small chord along the Sun's southern region.      

Now, what does all this have to do with the question as to why November transits are more frequent than May transits?  

Well, first, if you do look at the graphics [www,usm.maine.edu/planet/da-7-december-2015]  you will remark that the November transit series has more members than the May transit series.   In fact, there are about twice as many.      We have to introduce one more moving part to understand this lack of parity.     A May transit occurs about a month after Mercury reaches aphelion, the point of greatest distance from the Sun.  When any planet is at its aphelion, it is moving most slowly in its orbit.     Conversely, when a planet is at its perhelion, its orbital velocity is at a maximum.   November transits occur a few days before Mercury reaches perihelion and therefore it is moving most quickly.       

We see that each transit in a series shifts either "northward" (November) or "southward" (May), because the transits happen at the ascending and descending nodes, respectively.   The shift of Mercury's position relative to the Sun will be maximum (about 200 arc-seconds) when Mercury is moving most slowly, or in May.   This shift will only be about 100 arc-seconds when Mercury is moving most rapidly, or in November.  

So, if you miss this next May transit, you'll have to wait until 2049 to see another transit in the month of May.

Yes, Earth transits are sometimes visible from other planets.    A while ago, we withdrew this same question from Pandora and will be lazy sods and repeat the answer: 
The last transit of Earth from Mars occurred on May 11, 1984.  The next transit of Earth from Mars won't happen until November 10, 2084.     Also, and quite excitingly, the Moon will also appear to transit the Sun, as well. It will either follow or precede Earth across the Sun.        If you miss the 2084 Earth transit event, you'll have to wait until November 15, 2163.     Transits of Earth from Mars are presently only possible in May and November.

The following two transits of Earth from Mars occur on May 10, 2189 and then again on May 13, 2268.

Transits of Earth from Jupiter are more common.  The last occurred on January 5, 2014.  The next won't happen until January 20, 2026.   Then, another occurs on June 24, 2055, followed by another on June 29, 2067.       

The next transit of Earth from Saturn occurs on July 20, 2020. The next transit of Earth from Saturn after the 2020 event occurs on July 16, 2049.