There are probably many factors coming to play there.

You have splinter cells (the crawl for ESB) says the Hoth base was a small group lead by Luke (which doesn't hold up with what we see... Leia and Rieekan seem to be doing the leading).

Mon Mothma, Madine and Ackbar don't seem to be at the Yavin base at all (you could argue that they were there but weren't seen but I doubt if with so much at stake they wouldn't be in the war room if they were on the planet).

It could be that Mon Mothma remained a prominent Senator until the Imperial Senate was disbanded in which case she may be in the process of being smuggled out of the capital during the attack on Yavin.

You also have the prospect of off world Alderaanians and refugees from other Imperial atrocities joining the fight later as well as whole worlds making a stand as the Imperial grip tightens (as Leia suggests to Tarkin).

It would be nice to carry more of the Yavin staff over to Hoth and have some others turn up in ROTJ:R as well as seeing the fleet at the end of ESB:R joined by similar fleets to form a giant armada in the last film to create the impression that the various Rebel factions have been driven to make one last desperate strike for freedom (which the Emperor has anticipated, prepared for and possibly encouraged).