(This took a lot of work, but I did it all while I was technically working so I don't mind as I got paid for it^^)
A list of all the times that a title wasn't defended at Wrestlemania and what the champion was doing at the time:
Wrestlemania 1 - 31.03.1985
+WWE Championship - Hulk Hogan - On card - Tag match with Mr. T
+WWF Women’s Tag Team - Velvet McIntyre & Desiree Petersen - Not on card - Was active on house shows
Wrestlemania 2 - 07.04.1986
+WWF Women’s Tag Team - Leilani Kai & Judy Martin - Not on card - Maybe in Japan
Wrestlemania III - 29.03.1987
+WWE Women's - Moolah - On card - Cornered Harley Race
+World Tag Team - Bret Hart & Jim Neidhart - On card - In six-man tag match with the crooked ref against Bulldogs & Santa who they screwed
+WWF Women’s Tag Team - Leilani Kai & Judy Martin - Not on card - In Japan
Wrestlemania IV - 27.03.1988
+WWE Women's - Sensational Sherri - Not on card - Was active on house shows
+WWF Women’s Tag Team - Jumping Bomb Angels - Not on card - Probably in Japan
Wrestlemania V - 02.04.1989
+All on card
Wrestlemania VI - 01.04.1990
+All on card
Wrestlemania VII - 24.03.1991
+All on card
Wrestlemania VIII - 05.04.1992
+All on card
Wrestlemania IX - 04.04.1993
+All on card
Wrestlemania X - 20.03.1994
+All on card
Wrestlemania XI - 02.04.1995
+WWE Women's - Bull Nakano - Not on card - Dropped title on Raw the next night before going to Japan
Wrestlemania XII - 31.03.1996
+WWE Intercontinental - Goldust - On card - In that terrible match with Piper that was taped outside
Wrestlemania 13 - 23.03.1997
+WWE European - British Bulldog - On card - Defended the tag title he held at the same time
Wrestlemania XIV - 29.03.1998
+All on card
Wrestlemania XV - 28.03.1999
+WWF Light Heavyweight - Gillberg - On card - In battle royal
Wrestlemania 2000 - 02.04.2000
+WWE Women's - Stephanie McMahon - On card - Cornered HHH in four-way WWF Championship match
+WWF Light Heavyweight - Dean Malenko - On card - In six-man tag vs Too Cool & Chyna
Wrestlemania X-7 - 01.04.2001
+WWF Light Heavyweight - Crash Holly - Not on card - Was on Raw the next night
Wrestlemania X8 - 17.03.2002
+WWE Cruiserweight - Tajiri - Not on card - Was on TV the next night
Wrestlemania XIX - 30.03.2003
+All on card
Wrestlemania XX - 14.03.2004
+WWE Intercontinental - Randy Orton - On card - n Evolution vs Rock ’n’ Sock 3-on-2 match
Wrestlemania 21 - 03.04.2005
+World Tag Team - William Regal & Tajiri - On card - Both in battle royal
+WWE Intercontinental - Shelton Benjamin - On card - In Money in the Bank ladder match
+WWE United States - Orlando Jordan - On card - In battle royal
+WWE Cruiserweight - Paul London - On card - In battle royal
+WWE Raw Tag Team - Eddie Guerrero & Rey Mysterio - On card - Faced each other in singles match
Wrestlemania 22 - 02.04.2006
+WWE Intercontinental - Shelton Benjamin - On card - In Money in the Bank ladder match
+WWE Cruiserweight - Gregory Helms - No on card - Injured (broken nose)
+WWE Raw Tag Team - Joey Mercury & Johnny Nitro - On card - Both in battle royal
Wrestlemania 23 - 01.04.2007
+World Tag Team - John Cena & Shawn Michaels - On card - Cena defended his WWE Championship against Michaels
+WWE Intercontinental - Umaga - On card - Vince’s pick vs Trump
+WWE Cruiserweight - Chavo Guerrero - On card - In lumberjack tag match with Helms vs Carlito & Ric Flair (dark match)
+WWE Raw Tag Team - Paul London & Brian Kendrick - On card - Lumberjacks
+ECW Heavyweight - Bobby Lashley - On card - Trump’s pick vs Vince
Wrestlemania XXIV - 30.03.2008
+WWE Women's - Beth Phoenix - On card - n tag match. Snoop Dog segment
+World Tag Team - Hardcore Holly & Cody Rhodes - On card - Both in battle royal
+WWE Intercontinental - Chris Jericho - On card - In Money in the Bank ladder match
+WWE United States - MVP - On card - In Money in the Bank ladder match
+WWE Raw Tag Team - John Morrison & The Miz - On card - Morrison in the Money in the Bank ladder match, Miz in battle royal
Wrestlemania XXV - 05.04.2009
+WWE Women's - Melina - On card - In battle royal
+WWE United States - MVP - On card - In Money in the Bank ladder match
+ECW Heavyweight - Jack Swagger - On card - Lumberjack
+WWE Divas - Maryse - On card - In battle royal
Wrestlemania XXVI - 28.03.2010
+WWE Women's - Michelle McCool - On card - In 10-man tag match
+WWE Intercontinental - Drew McIntyre - On card - In Money in the Bank ladder match
+WWE United States - The Miz - On card - Defended the tag title he held at the same time
+WWE Divas - Maryse - On card - In 10-man tag match
Wrestlemania XXVII - 03.04.2011
+WWE Intercontinental - Wade Barrett - On card - In 8-man tag with Corre
+WWE Raw Tag Team - Heath Slater & Justin Gabriel - On card - Both in 8-man tag with Corre
+WWE Divas - Eve Torres - Not on card - Was active
Wrestlemania 28 - 01.04.2012
+WWE United States - Santino Marella - On card - In 12-man tag match to decide GM
+WWE Divas - Beth Phoenix - On card - In tag match
Wrestlemania 29 - 07.04.2013
+WWE United States - Cesaro - Not on card - Was active
+WWE Divas - Kaitlyn - Not on card - Was active
Wrestlemania XXX - 06.04.2014
+WWE Intercontinental - Big E Langston - On card - In battle royal
+WWE United States - Dean Ambrose - On card - In six-man tag with Shield vs New Age Outlaws & Kane
Wrestlemania 31 - 29.03.2015
+WWE Divas - Nikki Bella - On card - In tag match
Wrestlemania 32 - 03.04.2016
+WWE Raw Tag Team - The New Day - On card - In six-man tag match (non-title) vs League of Nations
Wrestlemania 33 - 02.04.2017
+WWE Smackdown Tag Team - The Usos - On card - Both in battle royal
Wrestlemania 34 - 08.04.2018
+All on card
This list didn't turnout nearly as badly as I first expected when I worked out which titles were not defended. I think it's actually quite reasonable the majority of the time.
2005-2010 really stood out but a lot of those were Money in the Bank matches, which is a valid reason to not defend a title. There are a few times where tag champions held a second title and they only defended one, which is also fine. Along with some faction stuff (Evolution, Corre, New Day, Shield). And the Vince vs Trump match.