Oh, there are several reasons why I think so.
1) For me "LGBT" is just a shortend version of the much longer acronym which includes more Letters like "I" for Intersex or the "A" for aces.
2) People who are asexual and/are aromantic are surrounded by a society that tells them they have to be sexual, have an interest in sex to be called "normal", even though aces don't choose to be asexual! This is why they have to work out that they are asexual, it takes most asexuals quite some time to figure out what they are. In my opinion every identity (sexual, romantic and/or gender) that differs from the perceived norm (which is: being heterosexual and Cis) has probably to be worked out - and has a place within the LGBT.
3) Because Asexuality is perceived as abnormal by the majority, so aces get discriminated - which includes spo called "corrective rapes" to "show them they are just as (hetero-)sexual as everyone else.